Implement conditional loading of variables

[Angus Yeh]

It's okay to always load whatever variables directly obtained from listing tables etc, but some variables may require certain EOS/simulators etc to be valid, or meaningful.

I can probably do this by adding a conditional checking when loads these variables. The base t2model.Data class have default implementation always return True. The variable classes can elect overwrite the implementation, by checking if certain columns exists.

Examples includes those derived ones eg. Total Heat, which uses 'Generation rate' and 'Enthalpy' which is dangerous to assume they exist.

[Angus Yeh]

Added at commit ff0862a and fixed a few issues with it in a few following commits.