Handle multiclass verbs

[psfinaki]

So of course it turns out everything is not that easy with verb classes. Looks like some verbs effectively belong to two classes as they have two completely different parallel conjugations, e.g. sýpat or orat. A more common case though is when some forms have the only option and others have two, e.g. smát se.

Now, the situation with imperatives and verb classes is the following. If the verb has two classes (as per the third person singular), it does not guarantee that imperative will have options according to both classes. E.g. kapat has 2 third person singulars kape and kapá and only one imperative kapej.

The opposite also happens. A verb might have only one class as per the third person singular and at the same time can have two options for imperative as it was of multiple classes. E.g. plakat has only 1 third person singular pláče and two imperatives plač and plakej.

So the conclusion is likely that with multiple classes we cannot predict the imperative. So we either can have an option for verb class or an array of possible classes.

[psfinaki]

getClass function now returns int option and None gets serialized to an empty cell in the imperatives table. This variant is preferred over array of classes as presumably it will result in a more natural client code for getting verbs by classes.