re: Unidata#1996

Improve the error message and location that is reported when reading a filter with a variable that uses a filter that is not available on the reading platform.

This requires checking the availability of the filter, recording it, and failing when any attempt is made to read or write that variable. A test case was added for this in tst_filter.sh. Also, LOG level 0 message is generated giving the variable and the filter id.

Note that by design if there is no attempt to read or write the variable, then no error is reported; this means that, for example, ncdump -h will list the filter even though it is not actually available. This is important for allowing a user to see the filter details.