Cache some of the attributes of TimesModel class

[olejandro]

Currently we don't use model.data_years for any processing. It is only needed to generate a corresponding set for GAMS. I expect calling it once after all the processing is done should suffice. I guess we could cache the value?

Originally posted by @olejandro in #211 (comment)

[olejandro]

@siddharth-krishna would using @cached_property be useful here?

[siddharth-krishna]

Thanks, nice idea. Though if we are only calling it once, it shouldn't be necessary.

[olejandro]

Actually 3 times, I think:

• once to get data_years for export to dd
• once to determine past_years
• and once to determine model_years