Two CAMx readers

[zmoon]

Maybe we can combine them into one that lives at models/camx.py for v0.3

• https://github.com/noaa-oar-arl/monetio/blob/develop/monetio/models/camx.py (old one)
• https://github.com/noaa-oar-arl/monetio/blob/develop/monetio/models/_camx_mm.py (new one)

Sorry I spaced on this in #185 / #187 @blychs .

[blychs]

If we don't need to stick to the _model_mm.py naming convention for melodies monet, I am happy to do that.

[zmoon]

Yes for monetio v0.3 we plan to move away from that convention. Hopefully with tests (some _mm.py ones already have).