Extend `metacore` object by adding programming-related information

[kaz462]

To enhance the usage of metadata, what do you think of adding the following information to the metacore object?

1. a column of functions used for derivation in metacore$var_spec and metacore$value_spec
2. order of derivation column in metacore$var_spec and metacore$value_spec

Users can also add these two columns to the P21 Excel Spec (Variables, ValueLevel sheets), so that they can be read in through spec_to_metacore()

Thanks!

[statasaurus]

I really like this idea. The only thing I might change is to put the column of functions into the derivation table rather than the var_spec and value_spec so information isn't repeated. Also I would probably just put the derivation order in var_spec rather than var_spec and value_spec