Provide an id object to track site, camera, spectrograph, etc.

The image acquisition actors need to know their "site" in order to set the file prefix (PF$(site)A). Besides that, several actors need to know what camera/dewar/spectrograph they are connected to.

I think I'll make a pfscore module in tron_actorcore for these kinds of things. I'm using a SpectroIds class in the ccd actor which I'll move over for a start. By default the identity is picked up from the hostname (e.g. bee_r1.pfs), but can be overridden. The site cannot be deduced in that way, so right now I'm using a $PFS_SITE environment variable.