dettool -defineby... should require more things by default

[eugene]

dettool -defineby.. functions require only a minimal amount of fields to be set. Most of the restrictions (airmass, solangle, etc) are not normally going to be set, but a few should normally be set in some cases:

filter : any light-type of detrun (flat, fringe, etc?) should require a filter unless explicitly overridden (maybe with filter = any?).

time_begin : all detrend types should require a time_begin. even if a large range is desired, it is better to force the user to think about the lower bound.