Add column msLevel to the chromatographic peak table

[jorainer]

Add a column ms_level (or msLevel) to the chromatographic peak table of the XCMSnExp object. This is supposed to allow to distinguish between e.g. MS1 and MS2 detected peaks. This might be useful in future for SWATH analysis (see also issue #208).

[jorainer]

This would allow to perform peak detection on more than one MS level on the same XCMSnExp object with the identified peaks being added to the same peak matrix.

[jorainer]

For now we don't add this additional column. I keep the issue open in case we need at some point the possibility to have chromatographic peaks from multiple MS levels within the same XCMSnExp.

[jorainer]

This is now implemented and added.