Revise SeismicSources and SeismicReceivers

[bvanderbeek]

Cumbersome indexing of source/receiver coordinates. The id's can simply be stored in a vector and the coordinates should be a dictionary indexed directly by the id. The opposite is currently done (id's are a dictionary and coordinates are an array).

There may also be a type-issue with the statics dictionary in the future. While an Observable is typically a Float64 (or similar Number), this is not always the case. For example, SplittingParameters and PrincipalShearObservables store an observation tuple. However, as implemented, the static dictionaries only store a single type.