Added SMAC collective variable
Removed SwitchingFunction::inverse

Now cutoffs for Switching function are deterine using dmax
Added SMAC collective variable

In doing so I fixed a number of bugs in the KernelFunctions
class.  The derivatives calculated by the evaluate function
in this class were wrong.  Tests still need to be done to check
whether or not the derivatives are correct when non diagonal
metrics are used.
Added SMAC with planes
Fix for numerical derivatives of bridged vessels
Merge branch 'gradient' of https://github.com/plumed/plumed2 into gradient