Field Maps nan if spatial limits not set

[Laurie Nevay]

In a BDSIM format field map, if say xmin and xmax are not set, the range is 0 and nans appear in the field values resulting in infinite loops or a crash with Geant4. We should a) complain about this if the required information is missing and b) never pass nans to Geant4.

[Laurie Nevay]

Fix Issue #257. Warn and exit if the step size for an array is 0. This can happen when say the xmax and xmin variables aren't specified and default to 0.

→ <<cset 57b50f14eefa>>