CRAN release 2.2-7

• Added if(is.null(args$.combine)) args$.combine <- c to DEoptim.R to allow for optional custom .combine function.
• Set default mapping function to NULL. This improves performance by allowing us to avoid one R call during the optimization if there is no mapping function.
• Remove unused variable par in de4_0.c
• Remove unnecessary allocation of sexp_t_tmpC, sexp_gta_oldC in de4_0.c that led to stack imbalance, potential crash.
• Update comments in de4_0.c to reflect variable names that were changed when objects were converted from arrays to SEXPs.
• Refactor the loop that populates t_bestC from the initial population, so there's no longer the potential for t_bestC to be uninitialized.
• Fix call to parallel::clusterExport
• Use full names to avoid partial matching. Partial argument name matching could cause confusing errors if the user's objective function contains arguments similar to the MARGIN and FUN arguments in apply().
• Added tests/ directory; tests.R now tests for problems in argument names.
• Import from parallelly package and use parallelly::availableCores()
  instead of parallel::detectCores(). Thanks to Henrik Bengtsson for the suggestion.
• The parallelType argument to DEoptim.control() now may be specified using numeric values (as before), or using "none", "auto", "parallel", or "foreach". "none" corresponds to the numeric value 0, "parallel" with 1, and "foreach" with 2. "auto" will attempt to decide which package to use.