variables.csv: add column "typeStart"

[JuanLiOHRI]

There is one column in the current variables.csv about variable type: "typeEnd" (aka "variableType"). When working with real life data, it is possible to have other data types before data transformation and harmonization. I'd like to suggest adding a new "typeStart" column and likely some functions to transform data from their original form.

[yulric]

Is this issue still relevant @JuanLiOHRI ? If it is could you give an example of where this would be used. Information about start variables should be encoded in the variable details sheet using the typeStart column, I'm not sure if solves your issue.