flipr 0.3.1

• Remove any occurrence of base-R pipe operator in favor of magrittr pipe operator for now for backward compatibility with older versions of R.
• Improved way of handling statistics that rely on inter-point distances.
• Method $get_value() gains optional arguments keep_null_distribution and keep_permutations in case the user would like to keep track of either the permutation null distribution or the sampled permutations that have been used to produce it.
• Method $get_value() now handles extra parameters to be passed to the test statistics.