TwoPhotonSession to work with incomplete sessions (missing files)

[mitlabence]

It would be useful to make TwoPhotonSession flexible: e.g. to be able to work with it for recordings without LFP, without imaging, etc.
The class methods should throw an error if they need data that does not exist for the session (or was not provided).
Appropriate tests should be written, as this would be a critical element for all kinds of data analysis.