R6 PCA wrappers

[michaelweylandt]

Before we can do a full review, all the R stuff needs at least basic documentation and to be added to the NAMESPACE using roxygen2's @export directive. I'm not 100% sure what all needs to be exported on the R6 side but I'd guess Hadley's "R Packages" book will discuss what needs exporting (methods or just our pseudo-S3 helper functions).

[Banana1530]

Yes, documentations are key to the package. However, I thought it would be our focus in the third coding phase so I did not write docs for these wrappers.

Anyway, for the sake of this review, I added some basic documentations (they are far from the final version though). Also there are some use cases in tests suites. These should be enough for reviewing this PR.

[michaelweylandt]

Also, let me know if you want to rebase / clean up the commit history before merging.

[codecov[bot]]

Codecov Report

Merging #42 into master will decrease coverage by 0.24%. The diff coverage is 89.69%.

@@            Coverage Diff             @@
##           master      #42      +/-   ##
==========================================
- Coverage   90.22%   89.98%   -0.25%     
==========================================
  Files          30       32       +2     
  Lines        2016     2487     +471     
==========================================
+ Hits         1819     2238     +419     
- Misses        197      249      +52

Impacted Files	Coverage Δ
R/moma_arguments.R	92.13% <ø> (ø)	:arrow_up:
src/moma_level1.cpp	94.9% <100%> (+0.42%)	:arrow_up:
R/util.R	100% <100%> (ø)
R/moma_expose.R	93% <100%> (+0.21%)	:arrow_up:
R/sfpca.R	98.18% <100%> (ø)	:arrow_up:
R/moma_sfpca.R	88.74% <88.74%> (ø)
src/moma_solver.cpp	94.89% <0%> (-0.73%)	:arrow_down:

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