Default DF initialization

[sahil1105]

Provide an option in the %schema annotation to automatically instantiate a DF (of length 0) even if no such logical atoms appear in the output. Useful when dealing with parametrized programs where output may not include logical atoms of a certain relation but the subsequent code may assume it is there.

Could be flag in the %schema notation which is false by default (maybe default_init?). Could also be another annotation like %default_init [name of relation]. Schema is applied to column names if provided, else the standard x1,x2,.. is used.