Unify def* function checking behavior

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defData, defDataAdd, defRead, defReadAdd should all use the same code/ show same behaviour.

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#18

[assignUser]

This is one aspect of the function-based distributions (#75) that I have been thinking about, as it is not immediately apparent how to handle definition sanity checks. On the other hand, using valid R syntax would leave much of that work in R's hands.

The distributions would need to do most of the check when run for data generation (which is the case now for most things due to the ..var system.) So maybe not as big a problem after all ^-^

[assignUser]

#134