Inconsistent method names for setting relation inverse type

[lindyhopchris]

At the moment, you call $relation->inverseType('type') to set the inverse type for a non-polymorph relation. But for a polymorph, you call $relation->types('type1', 'type2') to set the inverse type. This doesn't feel intuitive or consistent.

The types method should just be the plural of the method that's used to set a single type. Or if it's possible just use a single method for both, e.g. inverse. (But need to check what method names are being used by the implementation to get the inverse type.)

[lindyhopchris]

Changing this to $relation->type('type') and $relation->types('type1', 'type2').

This means the getters are $relation->inverse() and $relation->inverseTypes().

For the moment I'll leave $relation->inverseType('type') but mark it as deprecated.