Delete mosaics

[vgeorge]

With the introduction of POST /mosaic it can be expected that some created mosaics might not be used. For example, the user might change their mind on the mosaic dates before running a prediction, leaving an unused mosaic behind.

I believe we need a DELETE /mosaic/:id endpoint that only admins can call and that returns an error if there is any timeframes using that mosaic.

Another useful endpoint would be something like POST /mosaic/purge that delete all unused mosaics.

This is not blocking any feature now; it is just a suggested enhancement.

@geohacker @ingalls @LanesGood please let me know if you have thoughts.