pade : Fix std::complex compil pb on clang.

- Fix since std::complex can NOT be used
for a generic type : cf :
http://en.cppreference.com/w/cpp/numeric/complex

"
The specializations std::complex<float>, std::complex<double>, and
std::complex<long double> are literal types for representing and
manipulating complex numbers.
The effect of instantiating the template complex for any other type is
unspecified.
"

So we added a new little gmp_complex class.

--> I can not yet remove blitz array because of
a macro ALL defined in blitz conflicting with H5 !!

To do when we get rid of blitz :

- port array (almost done in commented lines)
- change the gf.
- put into a namespace.
Fix problem when using existing boost

Because things were not in the good order in the main CMakeLists.txt
the pytriqs/boost link was not created on the first call of cmake
when an existing boost was used. It appeared on a second call though.

  modified:   CMakeLists.txt
Improve some aspects of documentation

Did an iteration over the installation part.

  modified:   doc/user_manual/install/Ubuntu.rst
  modified:   doc/user_manual/install/dependencies.rst
  modified:   doc/user_manual/install/wien2k_inst.rst
  modified:   doc/user_manual/wien2k/Ce-HI.rst
Fix in main CMakeLists.txt