More CSR methods

[david-cortes]

This PR adds upon the CSR methods:

• Fixes CSR slicing for RsparseMatrix types other than dgRMatrix (currently failing), and adds functionality to slice with sparse vectors as indices.
• Adds functions for rbinding CSR matrices and vectors in different formats.
• Adds more supported types in the conversion functions and registers them as coalesce method for some S4 classes.
• Adds a function to transpose sparse matrices by changing the storage order.
• Adds syntactic sugar for CSR matrices for more operators and functions that work only on the non-zero elements (e.g. sqrt(CSR)).

[codecov]

Codecov Report

Merging #63 (f9e8f3a) into master (aa27867) will decrease coverage by 1.19%.
The diff coverage is 69.65%.

❗ Current head f9e8f3a differs from pull request most recent head 697e768. Consider uploading reports for the commit 697e768 to get more accurate results

@@            Coverage Diff             @@
##           master      #63      +/-   ##
==========================================
- Coverage   74.55%   73.35%   -1.20%     
==========================================
  Files          14       14              
  Lines        1175     1385     +210     
==========================================
+ Hits          876     1016     +140     
- Misses        299      369      +70     

Impacted Files	Coverage Δ
R/methods.R	80.04% <69.65%> (-13.65%)	⬇️

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[david-cortes]

I'm starting to realize these PRs are getting too big and now it's hard to use ?rsparse::. Will move to a new package and report back once it gets into CRAN.