Add CASCADE option for drop_table

[dharvey-consbio]

Not as simple as it may seem. Requires something like:

from sqlalchemy.ext.compiler import compiles
from sqlalchemy.schema import DropTable

@compiles(DropTable, "postgresql")
def _compile_drop_table(element, compiler, **kwargs):
    return compiler.visit_drop_table(element) + " CASCADE"

https://stackoverflow.com/questions/38678336/sqlalchemy-how-to-implement-drop-table-cascade