Include 'MDC' as an option for coinc_far

There is a potential bug when handling MDC events and calling to calculate the coincidence FAR to have it fail due to MDC not being an option. This could be called from gwcelery at https://git.ligo.org/emfollow/gwcelery/-/blob/aebb33f48c7d691761519ce86e3957f25df5da89/gwcelery/tasks/raven.py#L49, where this would fail due to the code https://git.ligo.org/lscsoft/raven/-/blob/8451723565744f145d5c0048d3774f9751e59792/ligo/raven/search.py#L273

A simple fix would to treat MDC events as standard GRB and modify the line https://git.ligo.org/lscsoft/raven/-/blob/8451723565744f145d5c0048d3774f9751e59792/ligo/raven/search.py#L244 to include this.