Update Colvars library to version 2019-04-26

The following is list of relevant issues fixed and improvements:

Fix forces and missing output of runtime histogram for histogramRestraint
https://github.com/Colvars/colvars/pull/246

Use fix_modify to add configuration to Colvars:
https://github.com/Colvars/colvars/pull/216

Fix componentCoeff and name not working with orientationAngle components:
https://github.com/Colvars/colvars/issues/213

Fix 1-timestep offset with extendedLagrangian:
https://github.com/Colvars/colvars/pull/210

Changes to improve compiler support:
https://github.com/Colvars/colvars/pull/203

Fix ignored anisotropic cutoff3 for groupCoordNum:
https://github.com/Colvars/colvars/pull/202

New dipoleMagnitude variable:
https://github.com/Colvars/colvars/pull/198

Parser improvements:
https://github.com/Colvars/colvars/pull/196