Maybe `strict = True` should just be called `DBMS = True` (with the
default setting as DBMS = False)?
I we decide to separate that out I suggest to call the formats "IPAC"
and "IPAC/DBMS" to provide the context.
(Note that this distinction is irrelevant in reading. Every DBMS table
is also valid plain IPAC.)

Since the formats would only differ in when to raise an error, it would
be easy to inherit
```
class DbmsHeader(IpacHeader):
   write():
       do extra tests
       call IPAC write
```