WIP: Add support for sparse multivariate polynomials

This adds support for sparse multivariate polynomials. (As multivariate polynomials tend to be sparse I think that dense univariate and sparse multivariate are the most important types for polynomials) 

I'm not sure if there is interest in this functionality for this package, but I think it could be a good fit. I could also make a separate `SpareMultiPol.jl` package if that is preferred. 

The idea here would be that for instance the addition of two dense univariate polynomials with different variables would yield a sparse multivariate polynomial (not implemented).

Documentation is still missing but see `test/testmpol.jl` for some examples.