Adding a `SparseMatrix` to an `HMatrix`

[maltezfaria]

In some boundary integral equation applications, it makes sense to add a sparse correction matrix to an HMatrix approximation. In those cases, as it turns out, the sparse matrix only has non-zero entries precisely where the HMatrix has dense blocks, so adding the two should be rather simple and cost-effective (i.e. no increase in the rank).

• [x] Add an extension where, if SparseArrays is loaded, the axpy! or axpby! function is defined to add a sparse matrix in-place to an HMatrix
• [x] If the sparse matrix is nonzero in blocks where the HMatrix is low-rank, decide what to do

[maltezfaria]

The decision was to:

• Depend directly on SparseArrays, at least for now (no extension)
• Adding a sparse matrix to an HMatrix does not check that the sparse entries are inside a dense block, as that cannot be guaranteed even in the BIE example discussed above. It does check, however, that after the multiplication the memory of the resulting matrix is not much greater than before.