Matrix assembly rework

[JordiManyer]

This relates to what we discussed. @amartinhuertas @fverdugo

A little context:

The current implementation of the sparse matrix assembly is (loosely) done as follows:

• when first assembled, we assemble entries InserterCSC object, with a similar layout to SparseMatrixCSC.
• when re-assembled, we assemble entries directly onto the sparse matrix.
  Both of these have the advantage of saving memory, but at the cost of doing sparse row/col index searches every time we assemble an entry.

We would like to investigate the option of working with COO-type structures. These are more expensive memory-wise, but can save quite a lot of time when re-assembling the matrix.

I've added a MWE on a possible implementation of the latter. Also, here are some results from the benchmarks at the end:

• When first assembling: Both approaches take the same time, but COO has 3x the memory footprint.
• When reassembling: COO is twice as fast than the current implementation.

[oriolcg]

Hi @JordiManyer , what is the status of this PR? Have you tried the new implementation for Nonlinear/transient problems?

[JordiManyer]

@oriolcg I don't have time atm, I'll try to push this further after the summer. I am not focusing complicated problems yet, but obviously anything that assembles repeatedly (like NL or transient) would benefit from this. I am worried about the memory footprint though... I think I can make most of it lazy so that we don't have to keep any Float values in memory, but it will take take some time to work it out.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.24%. Comparing base (9675711) to head (5486d72).
⚠️ Report is 1470 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1000   +/-   ##
=======================================
  Coverage   88.23%   88.24%           
=======================================
  Files         179      179           
  Lines       22593    22593           
=======================================
+ Hits        19936    19937    +1     
+ Misses       2657     2656    -1     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.