libsemigroups/libsemigroups: 2.1.4

This is a minor release with with a few minor changes: it is now possible to create the quotient_froidure_pin of a congruence with infinitely many classes; some exception messages were corrected for Bipartition objects; compilation of v2.1.0 to v2.1.3 did not work in cygwin for Windows; there was a file missing in the archive, which meant that it wasn't possible to build the documentation