Support searching by multiple platforms

[chuckwondo]

Currently, only a single platform may be supplied via the platform method. However, CMR supports searching for multiple platforms, so the platform method should be updated to accept either a single string or multiple strings, both for collections and granules, although this is not explicitly described for granules:

• https://cmr.earthdata.nasa.gov/search/site/docs/search/api.html#c-platform
• https://cmr.earthdata.nasa.gov/search/site/docs/search/api.html#g-platform