load spreadsheet data function

[DanudeSandstorm]

On initial loading, load only relevant spreadsheets to display page data, background load the remaining spreadsheets to reduce load time between pages (state changes)

Function in spreadsheet object which takes parameters of data to load and returns null until ready. Keeps track of what's been loading and what has a load request.

Optionally: Loads all data, but returns ready for what is asked.

[DanudeSandstorm]

Reopened as distinct issue. Background loading was never implemented in #12