Add support of distance function and flexible reference index for `DISE`

[FanwangM]

The original implementation does not support initial data points but only the medoid. Now we support

• [x] an integer
• [x] None, which uses the medoid
• [x] a list of integers

This is achieved by having a shared function called 'get_initial_selections'.

• [x] The original implementation did not support the assigning distance function, and the assigning flexible index is not supported. This PR addresses these two problems.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 92.40506% with 6 lines in your changes missing coverage. Please review.

Project coverage is 95.01%. Comparing base (d931154) to head (07f8abe). Report is 27 commits behind head on main.

Files with missing lines	Patch %	Lines
selector/methods/distance.py	92.40%	6 Missing :warning:

Additional details and impacted files

[](https://app.codecov.io/gh/theochem/Selector/pull/249?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=theochem) ```diff @@ Coverage Diff @@ ## main #249 +/- ## ========================================== - Coverage 96.00% 95.01% -0.99% ========================================== Files 9 9 Lines 975 1024 +49 ========================================== + Hits 936 973 +37 - Misses 39 51 +12 ``` | [Files with missing lines](https://app.codecov.io/gh/theochem/Selector/pull/249?dropdown=coverage&src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=theochem) | Coverage Δ | | |---|---|---| | [selector/methods/distance.py](https://app.codecov.io/gh/theochem/Selector/pull/249?src=pr&el=tree&filepath=selector%2Fmethods%2Fdistance.py&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=theochem#diff-c2VsZWN0b3IvbWV0aG9kcy9kaXN0YW5jZS5weQ==) | `95.12% <92.40%> (+2.52%)` | :arrow_up: | ... and [9 files with indirect coverage changes](https://app.codecov.io/gh/theochem/Selector/pull/249/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=theochem)