refactor c++ code:

[dselivanov]

move more functions to header files, change file naming

[codecov]

Codecov Report

Merging #55 (65ebbec) into master (fd41b3f) will increase coverage by 0.40%.
The diff coverage is 70.59%.

@@            Coverage Diff             @@
##           master      #55      +/-   ##
==========================================
+ Coverage   74.17%   74.58%   +0.40%     
==========================================
  Files          27       33       +6     
  Lines        2122     2105      -17     
==========================================
- Hits         1574     1570       -4     
+ Misses        548      535      -13     

Impacted Files	Coverage Δ
src/kmeans.cpp	0.00% <0.00%> (ø)
src/nnls.cpp	0.00% <0.00%> (ø)
src/rankmf.cpp	0.00% <0.00%> (ø)
inst/include/mapped_csr.hpp	53.84% <53.84%> (ø)
inst/include/wrmf_utils.hpp	84.37% <84.37%> (ø)
src/utils.cpp	79.31% <87.50%> (+0.90%)	⬆️
src/matrix_csr.cpp	93.07% <87.71%> (ø)
src/matrix_top_product.cpp	94.44% <88.88%> (ø)
src/factorization_machine.cpp	93.33% <93.33%> (ø)
src/FTRL.cpp	95.74% <93.75%> (+3.43%)	⬆️
... and 19 more

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