IGraph/M

Fixed:

• IGCompareCommunities had a corrupted usage message.
• Restored compatibility with older Linux distros that use GCC 4.8, such as RHEL / CentOS 7.
• The Fruchterman-Reingold, Kamada-Kawai and DrL layouts now verify that all weights are positive.
• IGAsymmetrixPreferenceGame misinterpreted the type weight matrix.

Other:

• Temporarily disabled resizing in graph editor, as it interferes with vertex creation.
• Documentation improvements.