PCHIP Interpolation for Sound Speed Profiles

[JPalmerK]

Hey there,
I'm working with glider data that has the benefit of collecting soundpseed profiles as it goes along it track. In working with these data I've found the need to use a different type of interpolation due to the disconnect between the water depth and the max depth of the glider. Furthermore, the documentation for Bellhop recomends using PCHIP for the bellhop model (HLS REPORT Report No. HLS-2021-01). For that reason I've created a modified version of uwpam.py that allows for a 'pchip' option for the soundspeed interpolation env['soundspeed_interp']. Below are the spline vs pchip interpolation results. Uploaded as a txt file.

Feel free to use as needed. I haven't updated the bokeh plots because I hate them. :)

uwapm.txt

[mchitre]

Thanks for the contribution. Would you like to PR this, or you prefer that I manually update this?