Update Colvars library to version 2020-01-27

One new feature (arithmetic path variables) and several smaller enhancements
or bugfixes.

    Fix error check when loading an old state file
    https://github.com/Colvars/colvars/pull/317

    Get default values for grid boundaries when available
    https://github.com/Colvars/colvars/pull/310

    Allow redefining existing index groups (warn when this happens)
    https://github.com/Colvars/colvars/pull/302

    Simplified replica communication syntax in metadynamics
    https://github.com/Colvars/colvars/pull/301

    Obtain the bias_energy for ABF biases
    https://github.com/Colvars/colvars/pull/294

    Fix reading path file with vector CVCs
    https://github.com/Colvars/colvars/pull/288

    Fix segfault at deleting CVBasedPath
    https://github.com/Colvars/colvars/pull/286

    Parrinello's (arithmetic) pathCV in CV space
    https://github.com/Colvars/colvars/pull/274

    Fix race condition when starting a bundle of walkers
    https://github.com/Colvars/colvars/pull/279