improve workspace's getDetector method

[Anders-Markvardsen]

While helping Matt it was found that running the following script

ws = mtd["PEARL00071180"]
inst = ws.getInstrument()

origin = ws.getInstrument().getSample().getPos()

for i in range(0, ws.getNumberHistograms()):
print i, " ", ws.getDetector(i).getTwoTheta(origin, V3D(0,0,1))*180/math.pi, " ", ws.getDetector(i).isMasked()

mask = []
for i in range(0, ws.getNumberHistograms()):
angle = ws.getDetector(i).getTwoTheta(origin, V3D(0,0,1))*180/math.pi
if angle > 97 or angle < 83:
mask.append(i)

MaskDetectors("PEARL00071180",WorkspaceIndexList=mask)

print "after masking"
for i in range(0, ws.getNumberHistograms()):
print ws.getDetector(i).getTwoTheta(origin, V3D(0,0,1))*180/math.pi, " ", ws.getDetector(i).isMasked()

generated the following error:

RuntimeError on line 7: "Instrument: Detector with ID 6 not found. search object" in file 'mask_detectors.py' at line 7

The problem is something along the lines of histograms in the workspace exists for which no detector exists

[Anders-Markvardsen]

This issue was originally trac ticket 3327