use dcorelib when triqs is not installed

[yomichi]

When TRIQS is not installed, dcorelib.triqs_compat will be used.
If you want to use dcorelib.triqs_compat when TRIQS is installed, set an environment variable DCORE_TRIQS_COMPAT=1

By default, use dcorelib.triqs_compat.
If you want to use TRIQS library, set an environment variable DCORE_TRIQS_COMPAT=0 explicitely.

[shinaoka]

We could remove the TRIQS detection, which may cause unexpected behavior (e.g., picking up an incompatible TRIQS version).

[yomichi]

OK, I changed to that to import triqs only when DCORE_TRIQS_COMPAT=0 is explicitly set, and I added a version check of imported TRIQS.