CONVERT: Add a parameter to permit export of tabular WCS via the TAB algorithm.

We apologise for the late running of this commit.  This change arose
out of Tim's desire to demonstrate our sparse data to Doug Tody under
the thread "Sparse cubes and FITS" back in March.  At that time I
wasn't sure that a decision for the way ahead had been made.

The USEAXIS parameter was being overworked.  This introduces a
separate parameter called ALLOWTAB (defaulting to TRUE) to control
whether or not tabular co-ordinates can be written using the TAB
algorithm.  Remove a redundant IF block, instead just not set the AST
TabOK attribute when ALLOWTAB is set FALSE.
CONVERT: Minor tidying.
CONVERT: Continue if COF_ISWCS returns bad status.

This arose while testing a round trip from a sparse NDF to FITS and
back to NDF.  CTYPEn with the TAB algorithm fail at a call to
AST_READ.  Since this means that the airlock WCS is unavailable, as
the algorithm involves a non-existent FITS extension, it is correct to
annul this error and set the GOTWCS flag to FALSE.