[WIP] interface dense matmul support in SparseDiffPy

[dance858]

Description

Please include a short summary of the change.
Issue link (if applicable):

Type of change

• New feature (backwards compatible)
• New feature (breaking API changes)
• Bug fix
• Other (Documentation, CI, ...)

Contribution checklist

• Add our license to new files.
• Check that your code adheres to our coding style.
• Write unittests.
• Run the unittests and check that they’re passing.
• Run the benchmarks to make sure your change doesn’t introduce a regression.

[github-actions]

Benchmarks that have improved:

   before           after         ratio
 [6301f44c]       [4ee12068]

•     1.39±0s          1.15±0s     0.83  gini_portfolio.Cajas.time_compile_problem
  

Benchmarks that have stayed the same:

   before           after         ratio
 [6301f44c]       [4ee12068]
      246±0ms          252±0ms     1.02  gini_portfolio.Murray.time_compile_problem
      1.03±0s          1.04±0s     1.01  finance.FactorCovarianceModel.time_compile_problem
      135±0ms          136±0ms     1.01  high_dim_convex_plasticity.ConvexPlasticity.time_compile_problem
     41.7±0ms         41.9±0ms     1.01  matrix_stuffing.SmallMatrixStuffing.time_compile_problem
      13.7±0s          13.8±0s     1.00  finance.CVaRBenchmark.time_compile_problem
      2.56±0s          2.57±0s     1.00  quantum_hilbert_matrix.QuantumHilbertMatrix.time_compile_problem
      4.15±0s          4.14±0s     1.00  huber_regression.HuberRegression.time_compile_problem
      22.3±0s          22.2±0s     1.00  sdp_segfault_1132_benchmark.SDPSegfault1132Benchmark.time_compile_problem
      5.45±0s          5.42±0s     1.00  optimal_advertising.OptimalAdvertising.time_compile_problem
      941±0ms          937±0ms     1.00  simple_LP_benchmarks.SimpleScalarParametrizedLPBenchmark.time_compile_problem
      291±0ms          289±0ms     0.99  matrix_stuffing.ParamSmallMatrixStuffing.time_compile_problem
      1.88±0s          1.87±0s     0.99  simple_QP_benchmarks.UnconstrainedQP.time_compile_problem
      337±0ms          334±0ms     0.99  gini_portfolio.Yitzhaki.time_compile_problem
      1.49±0s          1.48±0s     0.99  matrix_stuffing.ParamConeMatrixStuffing.time_compile_problem
      721±0ms          712±0ms     0.99  matrix_stuffing.ConeMatrixStuffingBench.time_compile_problem
      755±0ms          743±0ms     0.98  simple_QP_benchmarks.LeastSquares.time_compile_problem
      4.76±0s          4.66±0s     0.98  svm_l1_regularization.SVMWithL1Regularization.time_compile_problem
      261±0ms          254±0ms     0.97  simple_QP_benchmarks.SimpleQPBenchmark.time_compile_problem
      1.62±0s          1.58±0s     0.97  tv_inpainting.TvInpainting.time_compile_problem
     16.1±0ms         15.5±0ms     0.96  simple_LP_benchmarks.SimpleFullyParametrizedLPBenchmark.time_compile_problem
      587±0ms          565±0ms     0.96  semidefinite_programming.SemidefiniteProgramming.time_compile_problem
      293±0ms          282±0ms     0.96  slow_pruning_1668_benchmark.SlowPruningBenchmark.time_compile_problem
      10.6±0s          10.2±0s     0.96  simple_LP_benchmarks.SimpleLPBenchmark.time_compile_problem
     16.4±0ms         15.4±0ms     0.94  simple_QP_benchmarks.ParametrizedQPBenchmark.time_compile_problem