Handle dates in R environment

[amattioc]

Currently date and time_period objects are not managed by the R environment. We should consider adding support for dates at least for dates:

• from R to VTL: date classes in datatset components shoud be marshalled into VTL dates (YYYY-MM-DD) (this could be a bit tricky as JRI seems not to support dates natively, but it should be feasible anyway). Only date classes will be supported, so it will be up to the user to perform conversins before entering VTL.

• from VTL to R: currently dates and time_period is cast to string because even jdx does not support dates. It should be possible to format everything to a standard R date format (YYYY-MM-DD) and set a flag somewhere that implies recasting once in R (as.Date should be sufficient). An example of flag could be an attribute (e.g. date_columns) that lists all the date components