Altitude lookup table

Currently, altitude for pvlib.location based algorithms defaults to zero, but if we include a low-resolution altitude lookup, we can provide better results when altitude is not specified. We can make this altitude lookup the same format as LinkeTurbidities.h5, so it wouldn’t require that much new code or any new dependencies. I was able to build an altitude map using open data aggregated by tilezen. My test H5 file is currently 13 mb using 4320x2160 resolution, uint16 altitude, and gzip compression. We are free to distribute this data, but we do need to do is add this attribution somewhere in the documentation. Would you guys be interested in this feature? Should I make a pull request?

Here is a plot of my sample :

[AdamRJensen]

I think it’d be great to have the option of something like this:

pvlib.location.Location(latitude=55, longitude=10, altitude='SRTM')

and then have the location object call an API (using a function from iotools) that retrieved the elevation.

[adriesse]

I think this is a nice addition. There are probably a variety of technical variations possible, for example using netcdf, but it’s also fine the way it is.