Switch to MatrixExtra

[david-cortes]

Package MatrixExtra is now on CRAN and has passed most of the CRAN checks:
https://cran.r-project.org/web/checks/check_results_MatrixExtra.html

This PR removes the functionality which is now in MatrixExtra, importing the package instead. I don't know which operators (if any) specifically are required, so I added it under Depends so as to load the whole namespace.

[codecov]

Codecov Report

Merging #68 (9cf18c2) into master (6e72fed) will decrease coverage by 4.22%.
The diff coverage is 73.33%.

❗ Current head 9cf18c2 differs from pull request most recent head 07bd49f. Consider uploading reports for the commit 07bd49f to get more accurate results

@@            Coverage Diff             @@
##           master      #68      +/-   ##
==========================================
- Coverage   74.87%   70.65%   -4.23%     
==========================================
  Files          14       13       -1     
  Lines        1194      995     -199     
==========================================
- Hits          894      703     -191     
+ Misses        300      292       -8     

Impacted Files	Coverage Δ
R/model_WRMF.R	75.95% <73.33%> (-0.91%)	⬇️

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