Finer-grained control of trait validation

[eeg]

These requests pertain to the allowable values of particular traits. They apply when validating a CSV file that is uploaded, and they would also apply when, e.g., values might be input through a web form in the future.

1. For numerically-valued traits (float or integer), it would sometimes be useful to limit the range of allowable values. For example, in addition to checking that the value provided is a number, also check that it is greater than zero.
2. It's great that multiple values can be provided for a trait, e.g., A|B. But for some traits, it would be better to disable that feature and allow only single values, A or B but not A|B.