read_grammatical_expressions will fail on grammars with lambda expressions

[Ashvin-Ranjan]

Grammars which use lambda expressions in some rules (such as the kinship grammar) will not load when read_grammatical_expressions is called upon their YAML file, with the error being that yaml.load cannot find [grammar name].<lambda>. It might be good to find a way to store these lambdas as removing them will cause load times to be increase as certain Meanings will need to be recomputed.