Extend support for GET TIME VALUE

[enekomartinmartinez]

I will soon start on adding support for Vensim's GET TIME VALUE function.
https://www.vensim.com/documentation/fn_get_time_value.html

[enekomartinmartinez]

Working on this.

[enekomartinmartinez]

It is complicate to include the full functionality of this function:

• Seems that Vensim has a bug on it, I am running it with Vensim 7.3.4 in Windows and I have the same results when relativeto is set to 0 an to 1
• In order to implement the measures 0 to 10 with relativeto 0 or 1 we need to parse the "Base date" which is included in the sketch

Therefore, for now I will only include measures 1 to 10 with relativeto 2 and measure 0 with relative to 0.

[enekomartinmartinez]

relativeto=2 has been fully implemented in branch dev.

In the future maybe some other users may want to implement the missing cases.