Ensure no replacement of PK columns

[willbryant]

Using a filter file with replace on a primary key column would not work. Users are perhaps not super likely to try this for a real explicit PK, but on tables that have no explicit PK they could easily do it for columns in a unique key that we happen to have chosen as the substitute PK. We'd better prevent this.

[willbryant]

Was closed by these commits, which had the wrong issue number:

• b592706 don't choose keys on columns with replace: filter as substitute PKs (closes #58)
• 0be5310 check there's no replace: filter on explicit primary key columns (#58)