Need a way to construct layer strings

[dgarciabriseno]

Layer strings tell API endpoints which telescope and instrument to use, for example 13 => SDO AIA 304
We'll need a mapping of these, maybe an enum for users to select data sources.

Also Layer strings can have multiple sources so that the final image is a composition of the given layers.
i.e. [8,1,50],[13,1,50] will return a composite of AIA 94 and AIA 304 each with 50% opacity layered on top of each other.