Remove .sdf assumption in GAIA

[timj]

With commit ed5ae8dca994dbc85d01337422a1e17ffbc2ac17 GAIA no longer assumes .sdf in the C and Fortran code. The TCL code is full of .sdf though and this needs to be cleaned up. Not sure how best to refactor the code at the moment (Tcl interface to dat_par.h? Tcl library that at least defines the file extension in a single place in GAIA?). @pwdraper how would you approach it?

This would be relevant if someone is writing an HDS emulation library on top of HDF5 and that person thinks it would be confusing to continue to call the files .sdf.

[pwdraper]

The obvious way to handle this in Tcl is to use a global variable. It would be easy to add this into the Gaia package init section, see the gaia/generic/Gaia.C file (in fact there is a Hds_Init section that looks like they would be better places to put this).