#292 Normaliser should check existance of ancillary applications

[John]

I just ran a couple of jobs through Digital Preservation Recorder - Preservation.

Because the workstation had never been used before, no xena preferences were set. This meant that when Normalisation was attempted, the normalisers could not do anything much.

It would be useful if, when the normalisers (are loaded and reported in the console) some indication of the fact that the normaliser is not going to work. Would have saved a lot of time by not having to restart the step.

In this case, things like tesseract, ImageMagick and libpst were not even installed on the machine

How to ensure that the relevant helper applications are available and the paths are set correctly on all platforms is left as an exercise for the programmer.