Monte Carlo is used as the general (fallback)s method. Also, code for
the triangular mesh case is included, but unused. The way we create
surfaces now doesn't produce consistent surface normal directions. This
makes the triangular method fail.

Re #19154

This allows for `material` not to be involved in the majority of
mantid source files.

User input could mean that the assert is not always true.

Avoid calls to exit that could cause the program to just stop without
any warning.

Refs #16044

Assign the correct rule number when passing a single rule to
Object::procPair. The method detected that only one rule was present
correctly but failed to tell the external code which one it had found.
This led to the rule map being updated incorrectly.

The commit also switches the order in which the rules are assigned in
Intersection constructor to be consistent with the rest of the class
and the Union operation.