Fix Column and metadata in SQLModel Generator

[THUzxj]

I encountered a similar problem with #302 and I tried to fix it, including

• Force to use Column instead of mapped_column, through passing is_table=True
• Modify generate_base to generate metadata variable if Model exists

Maybe we can also refactor generate_models to decouple the function of getting models and collecting imports.

It is probably just a temporary solution to the issue. Thus it's up to you whether to merge. Thanks.

[coveralls]

coverage: 99.035%. remained the same
when pulling d1fec15 on THUzxj:master
into 6b8bc90 on agronholm:master.

[agronholm]

I am working on a proper fix for the problem. Most of the time, Column should not be used by the generator.