Add nnz and matrix memory size to index

[sandorkertesz]

With this PR a given matrix entry in the index file contains two additional keys:

• nnz: number of nonzero elements in the sparse matrix
• memory: the estimated size (in bytes) of the matrix in memory when loaded with scipy.sparse.load_npz()

This information will be used to estimate the matrix size before loading. It will be utilised in the yet to be added in memory matrix cache.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.03%. Comparing base (8cb012a) to head (914f19e).

Additional details and impacted files

@@           Coverage Diff            @@
##           develop      #45   +/-   ##
========================================
  Coverage    92.03%   92.03%           
========================================
  Files            7        7           
  Lines          251      251           
  Branches         7        7           
========================================
  Hits           231      231           
  Misses          18       18           
  Partials         2        2           

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

---

🚨 Try these New Features:

• Flaky Tests Detection - Detect and resolve failed and flaky tests