TEMPO-format observatories must have a TEMPO site code to find clock corrections

[aarchiba]

The way TEMPO stores its clock corrections is all in one big file (possibly split via INCLUDE statements) with each clock correction labelled with the site code. Currently if PINT is given an observatory without a site code it just loads all the clock corrections for every telescope and reports no error, but obviously gives a totally bogus set of clock corrections. The problem is that tempo site codes are used as aliases, so you can't afford to have, say, the GBT-old-position site use the same site code as the GBT-current-position, but without that how do you find a set of TEMPO-format clock corrections for it?