Change META Home "Method of vent. rate determination" to dropdown

[gabroko]

Allow entries in META Home's "Method of vent. rate determination" only to be from the list in definition (same as in META Room).
Update "validate" function to check those META Home entries

[retostauffer]

@gabroko Thanks for opening the issue.

I (only on my local copy) added the same "validate" rules to META-Home as the one we use in META-Room.

Status

• In both cases, this is a non-required (non-mandatory) field, the user can leave it empty.
• I also do not validate (check) what's in there in our R package.

Options

I see two options:

1. We leave it as is (XLSX validates, R does not care)
2. We could implement a check in R (annex_validate()) to force the user to either
   • Leave it empty (as it is a non-mandatory field)
   • If not empty, it must be one of the values in the definition table (ensure the user does not overrule the XLSX validation)

What should we go for?