Add function to retrieve example datasets

It would be nice to have a clean way for gallery examples to get access to the test data files. See https://github.com/pvlib/pvlib-python/pull/860#discussion_r379847062

For a function like load_dataset('greensboro-tmy'):

Pros:

• No need to monkey around with filepaths, especially ones that aren’t really meant to be public
• Associating files with keys means we can move and rename test data files without it being a breaking change
• Simplifies example code

Cons:

• The data files are in several formats (csv, json, nc, h5 etc), so this function would either have to know the appropriate reading method for each file (complicated) or just return a file handle and let the user parse the contents (less useful).

[wholmgren]

I would be happy if this

https://github.com/pvlib/pvlib-python/blob/c9929e85986a3368ed370203ef8699617c0cdc61/docs/examples/plot_greensboro_kimber_soiling.py#L39-L43

looked like

# get full path to the data file
file_path = dataset('greensboro-tmy')

# parse TMY3 data
greensboro, _ = read_tmy3(file_path, coerce_year=1990)

I don’t think the broader scope is feasible. To be clear, this is just something for the tests/examples - not for anything else.

metpy has a get_test_data function with the same idea, but a different implementation because it uses a caching back end that I think we should avoid.

example: https://unidata.github.io/MetPy/latest/examples/XArray_Projections.html#sphx-glr-examples-xarray-projections-py

[cwhanse]

+1 to having the function. I think a useful scope is to return the full path to the file. Reading file content and providing output in various formats seems like a large bite to chew.