Naming scheme for vector classes

It might be a good idea to decide on vector naming schemes. I think it would be important to have these schemes analog to what we settled on for matrices. Here are a few suggestions:

• Rename namespace vector to multivector. Then rename Vector class to Dense in analogy with matrix objects. The class will be called as multivector::Dense, i.e. this convention would include that using namespace multivector is never invoked.
• Alternatively, we could keep the namespace as vector and rename the class MultiDense, so the invocation of the type name would be vector::MultiDense. This name is a little bit cumbersome, but it also allows us to derive single vector from multivector class trivially and then have another type name vector::SingleDense. This may provide more flexibility.

Once we decide, it is straightforward to update names of classes and files.

@kswirydo @cameronrutherford @rothpc please chime in.