Fix bug in building regular type from python

- the flag is really enforce_copy and should force a copy
- for a view : it is false, no change
- for a regular type : it is true, and now this will enforce the
  copy in the call of numpy. numpy does it for us.
- The problematic case was when we construct a regular type
  from a complicated view in python, which is an array
  but is not C contiguous.
  (hence the PyArray_Check was false, and the C_Contiguous flag was not set)
  Now it is fine, since we ask numpy to systematically copy the data for us
  and build a C contigous array.
  --> the constructor from python does not support custom memory layout
  because numpy only support C and Fortran

Conflicts:

	triqs/arrays/impl/indexmap_storage_pair.hpp