Cyclotron trim coil has discontinuous derivative
follow up from issue #110 (closed) as discussed there.
As can be seen in the figure the derivative of the B field is discontinuous in the middle of the trim coil. This comes by construction of the calculation.
I think it would be good to:
 find out where the calculation comes from (see also comment #110 (comment 2391))
 get some measured trim coil profiles to see how important the effect could be.
Task list added 26 March (from #157 (comment 5500)):

PSItrim coils polynomial fit (with minimal order) (@frey_m) 
OPAL trim coil polynomial input (@snuverink_j) 
Comparison with current implementation for TC15, make polynomial input default (@frey_m, @snuverink_j) 
Documentation 
Regression/Unit test