Potential optimisation

Ported the SET instruction of the &OPTIMISE section of classic Garfield

https://garfield.web.cern.ch/help/garfield_57.html#Ref0589

to ComponentAnalyticField. For now, the target "function" is just a number (but this can easily changed if necessary) and the weights are set to 1. Also the field function does not include drift time, diffusion, gain, etc. at the moment.