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Add peak check

See merge request !10

This is nessesary to do proper testing of the new peak spec

Mass ordering check

Closes #4 and #8

See merge request !8

Mass ordering check omega check

See merge request !9

fix inheritance

This was causing a return value != 0 even when all interfields were
valid.

Revision date was (most likely) a legacy name that never had its
`lvcnrpy` integration test updated.

One addition convinience method to facilitate the modification of
datasets fields.

The two additional convinience methods facilitate the modification of
fields other than the (now) optional field derived field test classes
represent.