Support for multiple source insertion

[kyleaoman]

Inserting more than one source into a DataCube seems like a reasonable workflow to support. Need to consider best implementation. Some options:

• Martini accepts a list of sources instead of a single source and simply inserts each one in turn? Since iteration is over pixels, this is probably inefficient.
• Martini accepts a list of sources but combines them into a single source before inserting into cube. A CombinedSource module to facilitate this? Questions arise like what is the notional position or vsys of a combined source.
• Don't add any dedicated features but support with example workflows in documentation.