Conversion to Cartesian basis sets

[tovrstra]

(This was mentioned in #146.)

The conversion to Cartesian basis sets is needed for the WFN and WFX formats because these formats to not support pure functions. Just as in #258, this does not only require a change in the basis set definition, but also a transformation of all data expressed in the basis. At the lowest level, this can be implemented as function returning a transformed MolecularBasis instance and a transformation matrix. Also a utility function to transform everything in an IOData object (e.g. as a prepare function #191) would be useful. The transformation matrix is relatively easy to construct and is very sparse: block diagonal with blocks defined in iodata.overlap_cartpure.