Per guidance from Axel, revert lib/kokkos and lib/colvars to remove nullptr changes.
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
