minor changes to frontend
fix description of how to use sbt-idea

(it's an external plugin now no longer referenced
from within the sbt scripts)
cosmetic improvements in frontend
explaining formula
eliminate compiler warning
add __DATE__ and __TIME__ macros to xtc preprocessor
support simplify also using a feature model instead of just an fexpr context
changed handling of approx.fm and full feature model

Significant refactoring renaming the two into small feature model and
full feature model (explained in the comment in FeatureModelOptions.java)

Lexer and parser now also use the full feature model to prune the results
at the end. This means that the approx.fm does no longer need to be
complete, it is sufficient if it prunes the most common error paths