ONOFF does not report an error if Tcal doesn't exist

[wehimwich]

@vlbi-jun-yang reports that the onoff (and probably fivept) command does not report an error (and still runs) if the Tcal (or other RXG related parameters) are not defined for some detector. While in principle, Tcal doesn't have to be defined, it is probably reasonable to define one. Even a very trivial RXG file can do this. It may make sense to only issue a warning for this case. Likewise, the lo command could issue a warning if Tcal is not defined. Having an outright error and refusing to run onoff may be too strong of a reaction, but other opinions are welcome.