[algorithms.greedy] drop default for extension_algorithm

[ftalbrecht]

I ran into a situation where I was stupid enough to not give any extension algorithm to the greedy algorithm (thus ending up with the default trivial_basis_extension) which led to hard-to-interpret results. We discussed that defaulting to gram_schmidt_basis_extension might also not be a good idead (since one often wants to specify the product). Thus I propose to drop the default and force the user to explicitely set the basis extension algorithm.

[sdrave]

Done in 1dd4632.