Get arbitrary vars

[d-callan]

summary of changes:

1. add a getCollectionVariableNames method so we can get the colnames of variables in a collection
2. modify getCollectionData so that it accepts a character vector of colnames of variables it should return
3. add a method to help identify if a collection is 1:1 or 1:n w ancestors, so we know whether vars from this entity are merge-able w others

together these will let the mbio main package users ask for arbitrary variables from different collections by name, and get them back in a single data table