Bug "fixed" (i.e avoided).

Bug "fixed" (i.e avoided).

Bug description:
 - matitac (more precisely, MatitaEngine.assert_ng, hence the "include"
   command) parses files differently from Matita. In particular, it works on
   a camlp5 "Grammar.parsable" lexer, which is a lexer that remembers
   look-ahead tokens.
 - an "include" file can change the list of keywords for the lexer
   (e.g. when defining the "if-then-else" notation). Hence, after an include,
   the lexer to be used must change and thus the corresponding
   "Grammar.parsable" should change too. This was not the case since the
   "Grammar.parsable" was not regenerated to avoid loosing the look-ahead
   tokens

Bug avoidance:
 - we assume that the "include" command is properly terminated. Hence no
   look-ahead is performed. After each "include" command we regenerate
   the lexer to avoid the bug.

Note:
 - why don't we need to do this just after a "notation" command?
   Apparently, this is not required. However, I do not understand why.
   Is this to be better understood in the future?