parameters: minor changes

- lexical cast from string only for arithmetic type
or it does not compile (nor make sense).
-> extraction of an array was not tested, fine now.

- adding more cast operators to various integers.

- added collapsing of type when putting in the opaque object :
  at the moment, all integer to long.
  Parameters stores all integer types as long, and reconvert at the end.
  Rational : python only put long anyway ...

- update test

Conflicts:
	triqs/parameters/opaque_object_h5.hpp