draft for joss paper on sparse matrices

[jalvesz]

Opening stand-alone branch to manage joss paper

[jvdp1]

I am not familiar with this format. Overall LGTM. I just have a few suggestions.

[jvdp1]

Maybe for a new feature: call dense2csr(A, CSR)?

[jalvesz]

Yes, it could embed the call in just one. Actually I think it might be interesting to have a single interface to automatically detect the input and output matrices such as to internally apply the conversions having a unified API. Haven't thought it through yet though.

[jvdp1]

It could be something like:

    subroutine dense2sparse_${s1}$(dense,sparse)
        ${t1}$, intent(in) :: dense(:,:)
        class(base_${s1}$_type), intent(out) :: sparse

        select type (sparse)
        type is (COO_${s1}$_type)
        type is (...)

   end subroutine