format_sets: Parse different set formats into ngCMatrix format
Description
This function is intended to ensure proper formatting of the supplied sets to
the create_hierarchy constructor. Support for other input
objects can be created by writting a format_sets method for the class.
Usage
format_sets(x)
Arguments
x
The sets to be formatted
Value
A presence/absence matrix with sets as columns and elements as rows,
formatted as a ngCMatrix