store and load data per variable

[miserman]

As more variables are being added, it would be nice to not have to load all.

That would also remove the need for variable name encoding and time range offsets potentially.

Implementation

• will need to split data by variable, in addition to datasets
• will need to replace all current dataset loading mechanisms
• there is already a variable requesting mechanism that calculates summaries as needed, so might be able to add a loading mechanism to that

Stipulations

• will need some way to load multiple variables when requested for things like exports