Cleanup and small features

See merge request !54

Simplify property destruction logic and do not keep non-persistent props
around in clearVec(). If a prop is alive but non-persistent, someone
else has a shared_ptr to it, so we will not pull out the rug from under
their feet.

Properties: use partial template specialization.

See merge request !53

Avoid a lot of code duplication.

Also change constructor _def param to a const&, which was only in
the std::string specialization before, but makes sense everywhere.

Refactor using Entity tags

See merge request !52

Renamed T.cc files to T_impl.hh to get rid of IDE problems not showing them.

See merge request !51

PropertyPtr: clarify public interface by only exposing the necessary shared_ptr methods (cf #10)

See merge request !50

Delete broken operators

See merge request !49

Fix warnings

See merge request !48

Improve docs

See merge request !46

Type correctness

See merge request !45

Fix lifetime issues for Face::halfedges(), Cell::halffaces().

See merge request !44

Deprecate implicit conversion to int for handles

See merge request !42

FileManager: Avoid endless loop on invalid files

See merge request !41

Currently, the following API-usage is straight-forward and clean but
unfortunately wrong and dangerous:

`for (const auto heh: mesh.face(fh).halfedges()) { ... }`

This is because the temporary `Face` returned by `face()` reaches the
end of its lifetime, but `halfedges()` returned a reference to a vector that
lives inside that object. Reference lifetime extension does not apply
here.

This change splits behaviour so that in rvalue context (such as
temporaries), we move out the vector instead of returning a reference to
it.