Include instrument property in basic_instrument mock package

The previous absence of this keyword, which is usually present in the
defaul.yaml, caused a failure in scopesimple. There is no reason not to
include that keyword in the basic_instrument.

scopesim/tests/mocks/basic_instrument/default.yaml

@@ -14,6 +14,7 @@ yamls :
   - YAML_detector.yaml           # electronic and detector effects and properties
 
 properties :
+  instrument: "basic_instrument"
   psf_fwhm: 1.5             # [arcsec]
   modes : ["imaging"]       # default mode name
   dit: 60