"generated" as column/param flag

[Markus Demleitner]

Dear DAL, dear Registry,

In the TAP issue https://github.com/ivoa-std/TAP/issues/11, Pat and I
found that it would be really desirable to align the flags (indexed,
std, principal, primary, nullable) we have for columns (in
TAP_SCHEMA.columns) and TableParam-s (in VODataService tablesets).

The immediate occasion was that we should have a way to signal "What
you see on output in here is not necessarily what is in the database"
or, in effect, "you cannot write constraints against this column".
The classic use case from my end is that for most access_urls, DaCHS
stores a relative path and produces an appropriate URL based on where
it thinks it is running.  But there are many other use cases as well.

To address that, I am proposing a flag in the sense of TAP issue #11
for VODataService given it is nearing PR/RFC:
<https://github.com/ivoa-std/VODataService/pull/17>.

We were thinking about a good literal for this flag; I came up with
"unconstrainable", but that's a lot of letters and perhaps a bit
over-specific.  What I propose now is "generated".

So... what do people think?

Thanks,

          Markus