Change implementation of maximin support method to use flows instead of LPs

[DominikPeters]

A recent paper of Luis Sánchez-Fernández (https://arxiv.org/pdf/2309.15104.pdf) explains that the MMS voting rule can be computed via maximum flows instead of LPs. It could make sense for someone (e.g., a student) to implement this algorithm, for example using the networkx package.