Incorporating some changes mentioned in discussion #188/#185:

 - NumSum vs Sum
 - Added ValuesAll and ObservationsAll
 - Fixed TryGetObservation semantics
 - Added maskMissingFrom functionality
Removed dead tests
Fixed quickSelectInPlace name resolution
Attempt changes I mention in #189; testing how they affect our code base
Per #189, this seems to work best with our code base.
Fix fsproj references (for #189)
Opting for a little more flexibility in extracting typed columns per #189
Fixed spelling typo