Add getindex also for CachedBases on PowerManifolds

[kellertuer]

we have p[M,i] and X[M,i] for element access on power and product manifolds.

It would be nice to have the same for a cached basis B[M,i] which would boil down to calling get_vectors on that sub component.
Similarly one could also maybe

• introduce get_vectors to allow so access a range
• introduce B[M, i,j]for the jth basis vector of the component above
• maybe even B[M,i] as a shortcut for any basis (basically get_vector ?