Properly support accounts with more than one meter number

[DrDeke]

It would be a good idea to check for the presence of multiple meter numbers in the input file and either tell the user they need to select one of them, or compute separate totals for each meter number present.

[DrDeke]

I fixed the script so that it no longer gives nonsensical results when encountering more than one meter in the input file; it prints an error and exits if that happens. There is still work to be done to handle this more gracefully as described in this issue.