Update Colvars to version 2020-02-27

    This update contains several small new features or usability improvements.
    Descriptions and authorship information can be accessed from the pull
    requests listed below.

    Allow setting sigma parameter directly for metadynamics
    https://github.com/Colvars/colvars/pull/325

    Remove default values for lowerWall and upperWall legacy keywords
    https://github.com/Colvars/colvars/pull/324

    biasActualColvar option to bypass extended-Lagragian for a bias
    https://github.com/Colvars/colvars/pull/321

    Flexible restart-reading
    https://github.com/Colvars/colvars/pull/320