Numeral constants can be negative in Lustre input

Numerals are always positive, negative numerals have a unary minus
applied to their absolute values. Allow negative numerals as constants
in Lustre input.
Merge pull request #163 from chsticksel/develop

Numeral constants can be negative in Lustre input
Merge remote-tracking branch 'upstream/develop' into invGen
New invgen rules, termination check in lsd.