[dune-functions] SubSpaceBasis does not model the concept GlobalBasis

[Simon Praetorius]

Hi community,

I was wondering, is the SubSpaceBasis supposed to be a model of concept
GlobalBasis<GV>? It seems not to be the case and there is no test in the
testsuite. As far as I can see, the only thing missing is the update(GV)
method.

Best,
Simon