increase codecov to 100%

[erblast]

identify untested source code lines and add unit tests

cov <- covr::package_coverage()
cov
covr::file_report(cov, "R/simaerep.R") # generate codecoverage report for source code file
covr::percent_coverage(cov) == 100 # this tests whether code coverage is at 100%

add github action.

example:
https://github.com/IMPALA-Consortium/gsm.simaerep/blob/main/.github/workflows/test-coverage.yaml

[erblast]

open PR to branch 74_100perc_code_coverage

[erblast]

we might identify unnecessary code that can be deleted as well

[erblast]

code coverage for lint.R will be adressed here #75

[erblast]

code coverage increased from 94.3% --> 96.4%

still potential to increase for these files:

R/simaerep_plot.R: 95.71%
R/simaerep.R: 95.82%

[erblast]

code for parallelisation is not yet unit tested

a lot of it might be redundant when the next version of purrr is released

#80