don't check Vec/Mat item access in C++ by default

Some compilers (including gcc 4.2) seem to have difficulties to inline
functions with exceptions, even though it would be possible to know at
compile-time that the exceptions never get thrown.

The boundary checked versions for item access for Vec/Mat classes are
available as the VecX/MatX::At methods. The Python export has been
adapted to use the At methods, so guarantee we never crash from Python.