Support ExternalFile (CSV) in TableLookup / TableIndependentVariable

[jmarrec]

Enhancement Request

E+ has capabilities to point these objects to a CSV file, but OS SDK doesn't have it implemented.

We've added the fields but they are just reserved for potential future implementation at the moment. There is no getter/setter for it, nor a constructor that accepts an ExternalFile, and that's because we didn't add this capability when we replaced Table:MultiVariableLookup with Table:Lookup / Table:IndependentVariable (which didn't have the ExternalFile option) in #4652.

Detailed Description

Originally reported by @Ski90Moo at #5067

TableLookup: https://bigladdersoftware.com/epx/docs/23-2/input-output-reference/group-performance-tables.html#field-external-file-name

Table:IndependentVariable: https://bigladdersoftware.com/epx/docs/23-2/input-output-reference/group-performance-tables.html#field-external-file-name-1

Both have these fields:

[...]
  ,                        !- External File Name
  ,                        !- External File Column Number
  ,                        !- External File Starting Row Number
[...]


## Possible Implementation

Constructor that takes ExternalFile. Getters/Setters for Column Number, Start Row number.

Unclear how we enforce correspondance of ExternalFile between Table:Lookup and Table:IndependentVariable(s).