Replace asserts with errors, resolves #242

[akeeste]

This PR replaces all non-testing asserts with code that will raise errors. For the vast majority of asserts, the error messages are unchanged or have additions to say the parameter type or value that was given and is causing an issue.

Some try-except statements appeared like they should be errors, but just defined unused strings. I updated these to throw error messages.

Modules completed:

• [x] loads
• [x] power
• [x] utils
• [x] tidal
• [x] dolfyn
• river/io/d3d
• [x] river
• [x] wave/io
• [x] wave

I left river/io/d3d.py alone for now because #271 edits it and I wanted to avoid conflicts if possible.

Other items

• [x] @akeeste confirm that the new exceptions being thrown are caught by tests that evaluate incorrect input
• [x] I also changed a parameter in plot_directional_spectrum. It was using a parameter min that overwrote the built in Python function. i changed the parameter to color_level_min to prevent potential issues.

[ssolson]

@akeeste just checking if this is ready for another review

[akeeste]

@akeeste just checking if this is ready for another review

@ssolson Yes. I implemented all of the suggested changes. Let me know if there's any other changes you'd like to see.

I'm debating if the update to the min parameter of plot_directional_spectrum is necessary. How much of an issue is it to overwrite the python function min? Seems safer to not overwrite it, but the function call won't be backwards compatible then.

[ssolson]

I'm debating if the update to the min parameter of plot_directional_spectrum is necessary. How much of an issue is it to overwrite the python function min? Seems safer to not overwrite it, but the function call won't be backwards compatible then.

Its a very easy fix for the user to change a kwarg from min to color_bar_min and is the correct way to handle.

[akeeste]

Thanks @ssolson. I resolved those last few issues and will merge