Use MSnbase objects for storing CAMERA annotations

[stanstrup]

Related to adding annotation slot in XCMSnExp: sneumann/xcms#295
Related to finding where CAMERA uses XCMS1: #24 (comment)

Instead of CAMERA wrapping around the old xcmsSet object could it make sense to add CAMERA output to an XCMSnExp object instead?
I see two ways to do this:

1. A dedicated extension of the XCMSnExp object for CAMERA
2. A generic annotation slot that CAMERA could fill.

We also need to check how much of CAMERAs object really need to be kept around.
Also to consider is if there should be a ProcessHistory kept for the CAMERA processing.