v0.18.1

• bugfix: fallback to use a single core on windows-machines, fixing issue #131
• updating dependencies and required versions
• fix vignette due to updates in ptable-pkg
• simplify examples by using examplary ptable from ptable-pkg using ptable::pt_ex_cnts() and ptable::pt_ex_nums()
• feature: allow tabulation of non-perturbed variables in freqtab()
• feature: allow tabulation of non-perturbed variables in numtab()
• code linting
• bugfix: fixing issues with "simple" approach; harmonizing and code-cleanup