Lot of things can be const which makes the interface
a bit more legible and safe to use - lots of things are
accessing pointers that if they changed, would have wierd
and unpredicatable effects.
Probably of minor concern in practice, but a lot of strings
and vectors are being copied when they don't have to be.
Very crude profiling indicates this could save 30-50% of the time in
KIBIS::KIBIS after ParseFile, but this is incidental to
clarifying the API of the classes.
Report is const as it doesn't change the logical state of the
IBIS_ANY object (if m_reporter were a ref, it'd be mutable).
If a T* isn't null-checked inside a function, pass as ref
to show that the function expects and requires non-nullity.
