Better spreadsheet errors

[cliffckerr]

@robynstuart changes:

• checks the version of the spreadsheet to match optima; prints a BE warning if it doesn't match, or, if an exception is raised, prints the version mismatch along with the exception
• actually checks to make sure all parameters have been populated, and prints out if any haven't
• removes "pops" and "meta" from model-inputs.xlsx and instead just loads these explicitly (so now everything in there is actually a data input)
• gives a more helpful warning if too many rows, rather than just "pop from empty list"

not tested super extensively, but tests pass, and seems to give at least better errors with common spreadsheet errors (e.g. new or removed parameters).

[robynstuart]

looks good, thanks @cliffckerr