Fix #112 and put back g +=/-= matrix for imfreq

- The issue comes from the fact that the default generated
  += and co by the Python API is the one for immutable types, like int.
- Indeed, in python, for an int :
  x=1
  id(x)
  140266967205832
  x+=1
  id(x)
  140266967205808
- For a mutable type, like a gf, it is necessary to
  add explicitly the xxx_inplace_add functions.
- Added :
   - the generation of the inplace_xxx functions
   - a method in class_ in the wrapper generator that
     deduce all += operator from the + operators.
   - this assumes that the +=, ... are defined in C++.
   - The generation of such operators are optional, with option
     with_inplace_operators in the arithmetic flag.
- Also, added the overload g += M and g -= M for
  g : GfImfreq, M a complex matrix.
  Mainly for legacy Python codes.