Fix error when altering Part table that uses "master" keyword

[rly]

Fix #936.

The core of the issue appears to be that the "master" keyword in the Part table definition does not match the name of a table in context. So in this PR, I update alter only for Part tables so that "master" refers to the master table in context. There may be a more elegant way to fix this issue, but this seems to do the trick.

[rly]

This solution does not handle the case of a part table using the "master" keyword in a more complex way, though.

@schema
class Probe(dj.Manual):
    definition = """
    id: int
    """

    class Shank(dj.Part):
        definition = """
        -> master
        """

    class Electrode(dj.Part):
        definition = """
        -> master.Shank
        """

[rly]

Thanks for taking a look at this PR. This issue comes up in the Frank Lab often.

Syntax, integration, and style checks pass. Thanks for the comprehensive developer documentation!