support more schism variables

[wzhengui]

Hi All:

This PR includes the following changes.

1). improve the Thalassa UI by adding necessary widgets and removing unnecessary widgets
2). improve the Thalassa structure, which I thinks makes it more organized, more extendable to include other models. To add another model, one just needs: a). add datasets read method (utils.py), and b). add the corresponding plot method in "api.py"
3). resolve the loading issue for large-size file
4). support transformation of different projection
5). support to show more SCHISM variables (2D, 3D, and 4D variables).
6). support native SCHISM outputs (schout_*.nc). No need for pro-processing
7). Support triangle and quadrilaterals (quads are split into two triangles).

[saeed-moghimi-noaa]

I take a very fast look. I leave it to @brey and @pmav99 for final review.

[pmav99]

1. If you choose a netcdf with an unsupported format, there should be some indication on the UI that this specific file cannot be rendered and that you should choose a different one.
2. It is no longer possible to display the maximum elevation of the whole timeseries. I think that this is an import feature that needs to be addressed.
3. It should not be too hard to infer the actual "format" of the netcdf file. If that is the case, the UI can be simplified by removing the format combobox.
4. The layer combobox is not relevant for all the netcdf's variables. If the implementation is not too difficult, it would be nice if it was active only for the ones that need it and deactivated for the rest.

[pmav99]

Using this colormap throws an exception:

# ...
  File "/home/panos/.conda/envs/thalassa/lib/python3.8/site-packages/holoviews/plotting/util.py", line 912, in process_cmap
    raise ValueError("Supplied cmap %s not found among %s colormaps." %
ValueError: Supplied cmap jet not found among matplotlib, bokeh, or colorcet colormaps.

We should either add matplotlib to the dependencies or use a different colormap. I wouldn't add an extra dependency just for a colormap.

[wzhengui]

Hi @pmav99 : Thank you for reviewing the PR. I think these are good suggestions. I will try to address them.

[wzhengui]

Hi @pmav99 and @brey : Thank Panos' useful suggestions again. I am sorry that I didn't have a chance talking to George this morning, as I was working on Thalassa. I just finished the revision except point 4. I tried to dynamically update the interface according to different variable picked by users, but it turns out to be very tricky. I will keep this on my mind, and see whether this is a good solution.