Add option to constrain to increasing velocity models

[keurfonluu]

• Added: option increasing_velocity in evodcinv.EarthModel.configure to optimize for an increasing velocity model (addresses issue #13).
• Fixed: attribute xs was not correctly output by the method evodcinv.InversionResults.threshold.

Reminders:

• [x] Run invoke format to make sure the code follows the style guide,
• [x] Add tests for new features or tests that would have caught the bug that you're fixing,
• [x] Write detailed docstrings for all functions, classes and/or methods,
• [x] If adding new functionality, unit test it and add it to the documentation.

[codecov[bot]]

Codecov Report

Patch coverage: 100.00% and project coverage change: +3.28 :tada:

Comparison is base (0bce6f5) 49.74% compared to head (47604d4) 53.03%.

:exclamation: Current head 47604d4 differs from pull request most recent head 4c2fb02. Consider uploading reports for the commit 4c2fb02 to get more accurate results

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #14 +/- ## ========================================== + Coverage 49.74% 53.03% +3.28% ========================================== Files 17 17 Lines 599 626 +27 ========================================== + Hits 298 332 +34 + Misses 301 294 -7 ``` | [Impacted Files](https://app.codecov.io/gh/keurfonluu/evodcinv/pull/14?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Keurfon+Luu) | Coverage Δ | | |---|---|---| | [evodcinv/\_result.py](https://app.codecov.io/gh/keurfonluu/evodcinv/pull/14?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Keurfon+Luu#diff-ZXZvZGNpbnYvX3Jlc3VsdC5weQ==) | `20.20% <ø> (+2.02%)` | :arrow_up: | | [tests/helpers.py](https://app.codecov.io/gh/keurfonluu/evodcinv/pull/14?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Keurfon+Luu#diff-dGVzdHMvaGVscGVycy5weQ==) | `100.00% <ø> (ø)` | | | [evodcinv/\_model.py](https://app.codecov.io/gh/keurfonluu/evodcinv/pull/14?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Keurfon+Luu#diff-ZXZvZGNpbnYvX21vZGVsLnB5) | `75.52% <100.00%> (+4.42%)` | :arrow_up: | | [tests/test\_model.py](https://app.codecov.io/gh/keurfonluu/evodcinv/pull/14?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Keurfon+Luu#diff-dGVzdHMvdGVzdF9tb2RlbC5weQ==) | `100.00% <100.00%> (ø)` | |

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