Handle water layer

[keurfonluu]

• Fixed: allow the first layer to have zero or negative S-wave velocities (fixes issue #19).
• Fixed: number of parameters inverted display.

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

Attention: 5 lines in your changes are missing coverage. Please review.

Files	Coverage Δ
evodcinv/_layer.py	53.65% <ø> (ø)
evodcinv/_result.py	20.10% <0.00%> (ø)
evodcinv/_model.py	73.07% <55.55%> (-1.19%)	:arrow_down:

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