Source, DataCube objects can be in unusable state

[kyleaoman]

The pruning mechanism of source modules and the padding mechanism of datacubes mean that they can be put into a state where they should not be passed to martini. For example, once a mock is made with a source (it has been pruned in the process), applying a rotation and trying to produce a second mock of the same source should be invalid because it has lost its particles. Either the object should be flagged as no longer valid and martini modified to reject the source, or pruned particles should be kept aside (or masked) within the object and restored after use. Some treatment of padding of datacubes along the same lines would also be a good idea.