Change smrf.data class naming conventions

[jomey]

The files under smrf.data should be updated to match their classes they hold. Also rename the grid class, so we don't end up with references of grid.GRID

[scotthavens]

I think this follows the line of #163 as the class names are not all that useful. For example, air temperature is in air_temp.ta. Would this be better as air_temp.AirTemp or put the class names in the __init__.py to import cleaner?

[jomey]

We can combine these two tickets.

Only logged this separately as it looks like there could be some bigger restructuring of the load data logic once you open that can of worms.

I usually follow the pattern to put class names inside the __init__.py:
Example

[scotthavens]

As SMRF is growing with more and more subpackages, having class names in the __init__.py starts to make a lot more sense. While it sounds simple, this will be a pretty decent undertaking, especially cleaning up of the data folder!