Refactor write_evr to public API similar to read_evr and read_regions_csv

[ctuguinay]

Addresses #243:

The write_evr/evr_writer.py was refactored/removed so that reading a mask now goes through a similar workflow as .csv and .evr files, with read_mask taking in the xarray mask and creating a Regions2D object and this function is calling an underlying parser handling mask checking and contour extraction in regions2d_parser.py. Once the mask data is in Regions2D form, we can choose to save to either .evr or to .csv. Included is a simple values check that the mask is being correctly encoded. The test constructs a 4x4 mask and then within that box there is a smaller 2x2 box that is 1s (everything else is 0s) and then the Regions2D time and depth arrays are easy to check against.

[ctuguinay]

@LOCEANlloydizard @leewujung

Sorry, I know y'all are probably swamped with some things, but when one of you has time, could you take a look at this PR? I think this refactoring makes more sense than what I previously had which felt very disconnected from the other public API parts, but let me know if any of this doesn't make sense.

[leewujung]

Hey @ctuguinay : I like this change! It is much cleaner than having a flying function to write to evr.

The only question I have is when the mask has a hole - in this case does open_cv return 2 contours, one the outer contour and one the inner contour? If so, then in this case they will become 2 separate region_id entries in an Regions2D object, is that right?

[ctuguinay]

@leewujung Ah good point, there's a hierarchy structure to the contour format that I didn't encounter in my simple example. Let me figure that out and change how the opencv output is parsed and I'll add an example with a hole.

I'll also add an example with two patches that are separated (not contained within each other).

[ctuguinay]

@leewujung Once you get back, could we briefly talk about the inner/outer region case? I found strange contours in one of my test cases (potentially a bug in the in opencv??). A little hard to explain with this thread.

[leewujung]

Sounds good! Let's chat tomorrow!

[ctuguinay]

Also just noting that merging this will close #103