Improvements for random int/numeric generation

[Randy Syring]

I think MethodsMixin.testing_create() could be improved as follows:

• detect numeric column precision and/or Small/Normal/Big int columns and set random range accordingly
• read numeric range form the column's info:

# gives -100 to 100
db.Column(db.Integer, info={"random_magnitude": 100})

# gives -50 to 100
db.Column(db.Integer, info={"random_range": (-50, 100)})