REF: try using less memory in multipletests 'holms' see #1394
REF: reduce memory in fdrcorrection (minimal change)
REF: allow is_sorted in fdrcorrection see #1394
REF: add is_sorted option to fdr to avoid duplicate calculations
     delete intermediate arrays (all work except Hommel)
REF: multipletests: add is_sorted, performance: drop reverse indexing
     docstring
TST: multipletests: add tests for is_sorted
REF/DOC: use take (a bit faster), timing in docstring
Merge pull request #1396 from josef-pkt/multitest_memory

REF: Multipletests reduce memory usage