Concepts that specify the requirements for Green's function types.

The Green's function machinery is generic: any type satisfying the relevant concept can be used with the algorithms in Green's functions. The concepts in this group spell out the minimal interface those types must expose, most prominently triqs::gfs::MemoryGf, which constrains a type to own a mesh and a compatible data array.

Concepts
concept	triqs::gfs::BlockGf
	Concept checking that a type is a block Green's function.
concept	triqs::gfs::MemoryGf
	Concept checking that a type behaves like an in-memory Green's function.