New functions: psFitsIsImage and psFitsIsTable

[Paul Price]

We need functions that tell us what type of extension we're dealing with, based
on the header:

bool psFitsIsImage(psMetadata *header);
bool psFitsIsTable(psMetadata *header);

psFitsIsImage shall return true if the FITS header corresponds to that of a FITS
image. psFitsIsTable shall return true if the FITS header corresponds to that
of a FITS table.

I've added this function to the SDRS, and wanted to let you know.