Limit the number of spare caches kept. These are
used in speeding up lookup and depend on holding a
reference (handle) to the geometry.
Ignore the UseEverywhere flag in favor of using a
UgliGeometry to determine where there might be field.
If UseEverywhere is enabled then a single 2-D map
is used at all z locations.
- no test
- complain if problems
- complain and return B=0 if outside any steel z range
Force B=0 for positions beyond this z. This is useful for
UseEverywhere with 2nd SuperModule not energized.