Remove text referencing scalar grid types

[mcflugen]

What was described as a "scalar" grid type in the docs was really a variable without a grid. The correct way to deal with this case is to not assign a grid to these variables (i.e. their get_var_location should return "none"). None of the get_grid_ functions need to be implemented for these variables.

[PhilMiller]

This is something that would only really be appropriate for a new major version of the standard, because it would make previously-conformant models that return scalar for some variables non-conformant.

[mcflugen]

This is something that would only really be appropriate for a new major version of the standard

Yeah, I should probably be merging this into the develop branch. Originally scalar variables were intended to work this way (and do for at least some BMI models) but the docs didn't reflect this.