[fitsbits] BINTABLE/TABLE column count limitation

[Forveille thierry]

  I concur with many that we first of all need a description of actual use
cases, with considerably more motivation than someone coming up
against error messages, possibly after doing something that's quite far
from optimal (e.g. using many scalar columns for a vector).

Until/unless we get that, I'll consider that the discussion is of 
solutions to
a likely non-problem. Once/if we get that description, and in case the
problem actually exists, we'll be able to consider solutions and
weight their disruption to existing FITS readers against the
generality of the use case.