Add "period to date" to SQL browser

[Stiivi]

The feature "period to date" (PTD) requires proper implementation.

Description: to be defined.

[Stiivi]

Note: the original PTD was implemented using a hack – expr condition in the model that transformed into a SQLAlchemy construct using python's eval(). This caused unnecessary complexity in the code.

Better solution would be to have this recognized at the logical level and translated down to the physical using "legal" query building pathway.

[Stiivi]

Related cubes-discuss mailing list post with some thoughts.