[la.interfaces] some docu in VectorArray
[algorithms.basisextension] add pod_basis_extension
[algorithms.basisextension] add documentation
rename VectorArray -> VectorArrayInterface

Conflicts:

	src/pymor/algorithms/ei.py
	src/pymor/discretizations/nonlinear.py
	src/pymor/operators/basic.py
	src/pymor/operators/ei.py