Require NOT NULL on all primary key columns

[Azarattum]

Fixes #354.

This PR:

• adds not null check to is_table_compatible (updates error messages and checks for all columns)
• tests the new behaviour in test_is_table_compatible
• updates all the tests (c, rust, python) to use NOT NULL in table definitions
• updates example code in README to use NOT NULL

[jeromegn]

That makes sense!

[tantaman]

thanks!