Add method to multiply dense vectors with CSC matrices

[aaronquantexa]

Hi, I saw this issue #715 and thought I'd have a go seeing as it's a good first issue (over 1 year old!).

I've copied the style of code around the repo and lightly cloned this test: "SparseVector * CSCMatrix Lifted OpMulMatrix & Transpose" to validate the matrix multiplication is correct for DenseVectors.

To allow the multiplication I've extracted 2 conversions (DenseVector to CSCMatrix and CSCMatrix to DenseVector - this latter one has an assertion inside that the Matrix has just 1 row), which are public.

Do let me know if there is more testing / other changes required.

[dlwh]

Hi @aaronquantexa, thanks for the PR!

Two things:

1. I think it makes more sense to get there via conversion from the DenseMatrix/CSCMatrix implementation from here

   breeze/math/src/main/codegen/breeze/linalg/operators/CSCMatrixOps.scala

   Line 679 in a1c5646

   implicit def canMulDM_M[@expand.args(Int, Float, Double, Long) T]

   rather than the CSC/CSC, since a Transpose[DenseVector] is conceptually just a DenseMatrix with 1 row.

2. I'm in the middle of very large and obnoxious refactor to get Scala 3 support working, which has necessitated moving around a whole lot of stuff. If this doesn't merge cleanly into that branch, I may just copy and paste from your PR rather than merging, with due credit of course.

[aaronquantexa]

Cool, I'll have a go at using DenseMatrix.

On point 2, sounds fair - I'll leave this to you as maintainer.

[aaronquantexa]

If it doesn't merge easily, can remove the additional - now redundant functions - in DenseVector.scala

[jczestochowska]

related question: is subtraction between CSCMatrix and DenseVector supported? I tried - operator and all other related - operators but it seems like it's not implemented. @aaronquantexa @dlwh