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- Don't make a Copy method on FunctionInfo
- Have the Copy() methods on the TabulatedFunction subclasses return a pointer

necessitated creating a Copy method to do that on TabulatedFunction classes.

segfaults when you use the long-range dispersion correction with the
CustomNonbondedForce if the force uses TabulatedFunction objects. Basically, it
prevents the copy from taking ownership (and deleting) the functions in its
destructor.

The use cases for CustomNonbondedForce copying currently is to recompute the
long-range prefactor if the parameters change (so the CustomNonbondedForceImpl
kernels need a copy of the Force object). Doing a full (deep) copy would be a
bit slower (although would only need to be paid once for each
CustomNonbondedForce object being used).

from all .cpp and .h files.

This reverts commit 542ee547.

CustomNonbondedForceImpl long-range correction.

the square of the number of particles in the dominant LJ type overflows
integers.

Renamed methods for dealing with TabulatedFunctions to avoid problems in wrapper APIs.  Also fixed a few bugs.