change parallelization backend to 'parallel'
fix typo
fix imports to support 'parallel'
rework automatic execution of unit tests
adds fake vignettes to package
fix bug with rcpp and size_t
Merge branch 'master' into v2.2

Conflicts:
	DESCRIPTION
	NEWS
minor updates
Merge branch 'v2.2' into array_sumstats

Conflicts:
	.gitignore
	DESCRIPTION
	DESCRIPTION.template
fix infinite loop in Makefile