feat(utils): move channel flow utilities from modflow6

[wpbonelli]

Move open channel flow utility functions from MODFLOW 6 to new file flopy/utils/channel_utils.py. These were originally in modflow6's autotest/scripts/cross_section_functions.py.

Functions are included for

• calculating wetted perimeter and area from channel geometry and water surface depth
• computing discharge given channel geometry, flow depth, a roughness coefficient, and slope
• iteratively approximating flow depth given channel geometry, discharge, a roughness coefficient, and slope

Discharge can be calculated for rectangular channels given width and height, as well as for more complex channel cross sections given a set of contiguous segments making up the channel bed.

This PR follows up on #1621 with a view to moving modeling-related utilities to flopy

[codecov]

Codecov Report

Merging #1668 (1114c93) into develop (1114c93) will not change coverage.
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[jdhughes-usgs]

I am not sure anyone that uses flopy will need channel_utils but at least it is more available in flopy than in the mf6 repo.