spline2 MultiBsplines defined as PERIODIC return nonperiodic values beyond grid limit.

Client code should never ask for a value beyond the spline limit, but if they do the spline returns the limit value.
See
https://github.com/QMCPACK/qmcpack/blob/develop/src/spline2/MultiBsplineValue.hpp
Then look at
https://github.com/QMCPACK/qmcpack/blob/develop/src/spline2/MultiBsplineData.hpp
Splines that are not going to return values in line with their boundary conditions should throw an exception or assert.