Evaluate if users should be able to select correlation type in fmudesign

[berland]

Issue extracted from source code comment:

semeio/src/semeio/fmudesign/create_design.py

Lines 895 to 900 in 17bc020

	# Either do ImanConover followed by Permutation, or simply ImanConover
	if correlation_iterations > 0:
	# TODO: It is possible to let the user set the correlation type
	# if this is of interest. But for now we assume that users care about
	# pearson correlation, not spearman (rank) correlation.
	correlator = probabilit.correlation.Composite(