increase code coverage for the R-package

[mlampros]

I've added code coverage as discussed in #269. The only exception is the cleanup() method (either for a single estimator or for all estimators) which is tested (currently) on a unix-like OS. Feel free to close #269 if this PR works as expected.

One think that I've noticed is that the 'save_model' and 'dump_model' methods work only for RGF and not for FastRGF. thanks.

[StrikerRUS]

@mlampros Thank you very much for this PR!

Seems that two of new tests don't work on macOS:

* checking tests ...
  Running ‘testthat.R’ [22s/22s]
 ERROR
Running the tests in ‘tests/testthat.R’ failed.
Last 13 lines of output:
  > 
  > test_check("RGF")
  ── 1. Failure: the 'cleanup' method (ESTIMATOR specific) works as expected for b
  init_exists_upd_rgf && end_state_rgf && end_state_fastrgf isn't true.
  
  ── 2. Failure: the 'cleanup' method (APPLIES TO ALL ESTIMATORS) works as expecte
  &&... isn't true.
  
  ══ testthat results  ═══════════════════════════════════════════════════════════
  [ OK: 13 | SKIPPED: 0 | WARNINGS: 0 | FAILED: 2 ]
  1. Failure: the 'cleanup' method (ESTIMATOR specific) works as expected for both RGF and FastRGF (checking of the length of the '/tmp/rgf' default directory before and after the '$fit' method) (@test-RGF_package.R#501) 
  2. Failure: the 'cleanup' method (APPLIES TO ALL ESTIMATORS) works as expected for both RGF and FastRGF (checking of the length of the '/tmp/rgf' default directory before and after the '$fit' method) (@test-RGF_package.R#535) 
  
  Error: testthat unit tests failed
  Execution halted

[mlampros]

@StrikerRUS, I assumed that Mac osx will have the same temporary folder as on linux ('/tmp').
Is it ok if I restrict the cleanup() test only to the linux os? The cleanup() function is already tested on Python, and the R package doesn't do anything else than calling the python function from within R.

[StrikerRUS]

@mlampros I guess you can use tempdir() function as well as some environment variables to locate temp directory in a cross-platform way and don't hardcode it.

[mlampros]

@StrikerRUS,
thanks for make me aware of the one-liner dirname(tempdir()),
I think it will do the work. If not then I'll use the accepted answer of the second web-link
that you've mentioned.

[StrikerRUS]

@mlampros Thank you! Please check some my comments about tests below.

[fukatani]

@mlampros Sorry for late response. Are you debugging failure on appveyor?

[mlampros]

@fukatani, I'll add the remaining tests for this PR tonight. I think, the appveyor error is a connection failure, so with a re-run it will disappear. thanks.

[StrikerRUS]

@mlampros Thank you very much for your hard work! I left some minor comments below.

[StrikerRUS]

Are these duplicated lines expected or it is a bug?

[mlampros]

I'm not sure. The previous .Rd file consisted of a single line. The .Rd file is the manual page of the corresponding .R file. It has to do with the roxygen2 package and how it builds the manual pages from the .R files (I've used the roxygen2 7.0.2 version for this PR)

[fukatani]

This line is needed?

[mlampros]

The previous .Rd file consisted of a single line. The .Rd file is the manual page of the corresponding .R file. It has to do with the roxygen2 package and how it builds the manual pages from the .R files (I've used the roxygen2 7.0.2 version for this PR)

[fukatani]

LGTM excepts above the comment.