Check if table is ABS time series spreadsheet

[MattCowgill]

New users are often tripped up by the distinction between ABS time series spreadsheets and 'data cubes'. This is not helped by the fact that the ABS de-emphasises this distinction on the new website.

Add a new function that checks if a spreadsheet is an ABS time series spreadsheet. Throw an informative error if attempting to use read_abs() or read_abs_local() with a data cube.