Refactor Data API

[micahpw-nrel]

• One method per schema returned.
• One method per base table returned (population, hdi, gdp, etc.)
• One method per weather variable data returned (?)
• [ ]

[elainethale]

Regarding the comment https://github.com/dsgrid/stride/pull/10/files#r2280939321 that I think the first item is addressing, I like that there's just one get_load_duration_curve function, for example. Maybe it could just always return a DataFrame that includes year and scenario columns, at least by default (and thus keep the schema the same)? Maybe there could be an optional argument to drop trivial columns that defaults to False (to better support applications in which the function is always used in the same way to return data for one year or one scenario)?