Update Colvars to version 2019-08-01

One new feature (geometric path variables) and bugfixes.

Minimize occurrence of file-read errors in multiple-walker metadynamics
https://github.com/Colvars/colvars/pull/276

Better error messages for RMSD
https://github.com/Colvars/colvars/pull/272

Ensemble-biased metadynamics documentation
https://github.com/Colvars/colvars/pull/261

Fix bug on multiple walkers metadynamics, pmf included twice
https://github.com/Colvars/colvars/pull/259

Implementation of the geometric path collective variables (C++11-only feature)
https://github.com/Colvars/colvars/pull/249