add custom easyblock for reticulate R package

[smoors]

(created using eb --new-pr)

see also #2665 (review)

[smoors]

Test report by @smoors

Overview of tested easyconfigs (in order)

• SUCCESS R-4.1.0-foss-2021a.ebtest

Build succeeded for 1 out of 1 (1 easyconfigs in total)
node310.hydra.os - Linux CentOS Linux 7.9.2009, x86_64, Intel(R) Xeon(R) Gold 6148 CPU @ 2.40GHz (skylake_avx512), Python 2.7.5
See https://gist.github.com/e40c0e69c45038cd69038b1c6dbb1704 for a full test report.

[boegel]

lgtm

[boegel]

Test report by @boegel

Overview of tested easyconfigs (in order)

• SUCCESS R-4.1.2-foss-2021b.eb

Build succeeded for 1 out of 1 (1 easyconfigs in total)
node3517.doduo.os - Linux RHEL 8.2, x86_64, AMD EPYC 7552 48-Core Processor (zen2), Python 3.6.8
See https://gist.github.com/652139cd7a5751a295097b43a1dcde21 for a full test report.

[boegel]

Works like a charm (as long as --parallel-extensions-install isn't used, see easybuilders/easybuild-framework#3959):

 $ ml show R/4.1.2-foss-2021b | grep RETI
setenv("RETICULATE_PYTHON","/software/Python/3.9.6-GCCcore-11.2.0/bin/python")

Thanks @smoors!