Predicate terminology is outdated

[goodmami]

PyDelphin still refers to "string" and "grammar" predicates, where these should be "surface" and "abstract" ("string" is compared to "type", not "grammar"/"abstract").

[goodmami]

This entails several steps:

• provide the correct terminology, but don't break backward compatibility
• issue a DeprecationWarning (see the @deprecated function decorator in delphin.util) if the old form is used
• add the affected functions/classes/etc. to Remove deprecated components #139

[goodmami]

@mcmillanmajora I had to double-check because "string" and "grammar" don't directly correlate with "surface" and "abstract", but it appears that the code was treating them as equivalent, so it should be safe to just rename one with the other. But in the interest of backward-compatibility, retain the old names as well. E.g.:

class Pred(...):
    ...
    ABSTRACT = GRAMMARPRED = 0
    ...
    @classmethod
    @deprecated(final_version='1.0.0', alternative='Pred.abstract()`)
    def grammarpred(cls, predstr):
        """Instantiate a Pred from its symbol string."""
        return cls.abstract(predstr)

    @classmethod
    def abstract(cls, predstr):
        """Instantiate a Pred from its symbol string."""
        lemma, pos, sense, _ = split_pred_string(predstr)
        return cls(Pred.ABSTRACT, lemma, pos, sense, predstr)
    ...

Also go through and reword docstrings, as appropriate, and unit tests.