Sets the matching algorith to use. A minimum weight perfect matching
(MINIMUM_WEIGHT_MATCHING) guarantees the 3/2 upper bound to the optimal
solution. A minimal weight perfect matching (MINIMAL_WEIGHT_MATCHING)
finds a locally minimal weight matching which does not offer any bound
guarantee but, as of 1/2017, is orders of magnitude faster than the
By default, MINIMAL_WEIGHT_MATCHING is selected.
TODO(user): Change the default when minimum matching gets faster.