[gfs] Fix issue #91

Python raises a ZeroDivisionError on floating-point division by zero,
which is a violation of IEEE 754. numpy.divide() obeys the standard
and only issues a warning (can be silenced).
[doc] Added example for delta function for GfImFreq, need to decide where it goes
[doc] temporary fix to double definition of frobenius norm
Merge pull request #154 from krivenko/new_converters

[converters] Two changes in Python converters
Merge pull request #149 from krivenko/issue91

[gfs] Fix issue #91
Merge pull request #111 from krivenko/operator_scalar_t

[1.2-dev] Better support for user-defined scalar_t in many_body_operator
Merge branch '1.2-dev' of github.com:TRIQS/triqs into 1.2-dev