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 Adding a constraint for velocity increasing with depth #13).
• Fixed: attribute xs was not correctly output by the method evodcinv.InversionResults.threshold.

Reminders:

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

[codecov]

Codecov Report

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

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

❗ 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

@@            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	Coverage Δ
evodcinv/_result.py	20.20% <ø> (+2.02%)	⬆️
tests/helpers.py	100.00% <ø> (ø)
evodcinv/_model.py	75.52% <100.00%> (+4.42%)	⬆️
tests/test_model.py	100.00% <100.00%> (ø)

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