Extending dimer coordinates for multimers

[psi-rking]

• [X] First test case for trimers (water trimer) working, with auto- and manual- coordinates.
• [X] Add user-specified interfrag coordinates test for water trimer (will test atom numbering)
• [X] Also, add test with reference points at COM and not at atom positions (will test linear combinations)
• [X] Test initial hessian computation (B-matrix is already tested; so this ought to be working)

[codecov[bot]]

Codecov Report

Base: 82.80% // Head: 83.16% // Increases project coverage by +0.36% :tada:

Coverage data is based on head (77c92f2) compared to base (87b74e6). Patch coverage: 78.29% of modified lines in pull request are covered.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #68 +/- ## ========================================== + Coverage 82.80% 83.16% +0.36% ========================================== Files 35 35 Lines 6576 6617 +41 ========================================== + Hits 5445 5503 +58 + Misses 1131 1114 -17 ``` | [Impacted Files](https://codecov.io/gh/psi-rking/optking/pull/68?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Rollin+King) | Coverage Δ | | |---|---|---| | [optking/v3d.py](https://codecov.io/gh/psi-rking/optking/pull/68/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Rollin+King#diff-b3B0a2luZy92M2QucHk=) | `76.22% <ø> (-0.17%)` | :arrow_down: | | [optking/optparams.py](https://codecov.io/gh/psi-rking/optking/pull/68/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Rollin+King#diff-b3B0a2luZy9vcHRwYXJhbXMucHk=) | `77.52% <17.64%> (+2.38%)` | :arrow_up: | | [optking/stre.py](https://codecov.io/gh/psi-rking/optking/pull/68/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Rollin+King#diff-b3B0a2luZy9zdHJlLnB5) | `71.57% <58.33%> (+0.98%)` | :arrow_up: | | [optking/optimize.py](https://codecov.io/gh/psi-rking/optking/pull/68/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Rollin+King#diff-b3B0a2luZy9vcHRpbWl6ZS5weQ==) | `86.19% <66.66%> (-0.05%)` | :arrow_down: | | [optking/displace.py](https://codecov.io/gh/psi-rking/optking/pull/68/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Rollin+King#diff-b3B0a2luZy9kaXNwbGFjZS5weQ==) | `86.95% <80.00%> (+0.05%)` | :arrow_up: | | [optking/dimerfrag.py](https://codecov.io/gh/psi-rking/optking/pull/68/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Rollin+King#diff-b3B0a2luZy9kaW1lcmZyYWcucHk=) | `86.70% <93.54%> (-0.57%)` | :arrow_down: | | [optking/IRCfollowing.py](https://codecov.io/gh/psi-rking/optking/pull/68/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Rollin+King#diff-b3B0a2luZy9JUkNmb2xsb3dpbmcucHk=) | `96.31% <100.00%> (ø)` | | | [optking/addIntcos.py](https://codecov.io/gh/psi-rking/optking/pull/68/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Rollin+King#diff-b3B0a2luZy9hZGRJbnRjb3MucHk=) | `78.34% <100.00%> (+1.96%)` | :arrow_up: | | [optking/exceptions.py](https://codecov.io/gh/psi-rking/optking/pull/68/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Rollin+King#diff-b3B0a2luZy9leGNlcHRpb25zLnB5) | `100.00% <100.00%> (ø)` | | | [optking/hessian.py](https://codecov.io/gh/psi-rking/optking/pull/68/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Rollin+King#diff-b3B0a2luZy9oZXNzaWFuLnB5) | `53.03% <100.00%> (+2.23%)` | :arrow_up: | | ... and [9 more](https://codecov.io/gh/psi-rking/optking/pull/68/diff?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Rollin+King) | | Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Rollin+King). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Rollin+King)

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