[Breaking] Make inverting a PDSparseMat throw an error

[ElOceanografo]

Alter the inv() method for PDSparseMat to fall through to the inv() method of its underlying matrix. If that is a SparseMatrixCSC, an error will be thrown asking the user to convert to a dense matrix first (the default behavior in LinearAlgebra). This would address #87.

I considered writing a more PDMats-specific error message (e.g. "ERROR: The inverse of a sparse matrix can often be dense and can cause the computer to run out of memory. If you are sure you have enough memory, please convert your matrix to a dense matrix, for instance using inv(PDMat(Matrix(J)))". However, making the method fall through to whatever is defined for the underlying matrix type allows for the possibility someone will one day write a sparse matrix type that does have a sparse inverse. In that case, inverting a PDSparseMat wrapping such a matrix will "just work."