Add OpenMM CMAP support

[lohedges]

This PR adds support for CMAP terms in the Sire-to-OpenMM conversion layer. This turned out to be fairly straightforward, with the main complication being the different grid ordering used internally by OpenMM. The AMBER format stores the grid with phi varying slowest in column-major order, while OpenMM expects the grid with a half-period cyclic shift applied to both axes and the phi/psi axes swapped.

Unit tests confirm that a system containing CMAP terms converted to an OpenMM system with Sire gives identical energies to what would be obtained using the OpenMM Python API directly. A second unit test performs initial validation for the perturbable CMAP implementation by validating the protein CMAP terms are unmodified when changing lambda for a one-to-one mapping between residues in a protein backbone. (This also validates that the BioSimSpace CMAP merging is working correctly.) I'll add a unit test to BioSimSpace when I have an example protein mutation that involves CMAP terms.

The PR also closes #405 by adding a timeout to the GROMACS archive download in sire.load.

• I confirm that I have merged the latest version of devel into this branch before issuing this pull request (e.g. by running git pull origin devel): [y]
• I confirm that I have added a test for any new functionality in this pull request: [y]
• I confirm that I have added documentation (e.g. a new tutorial page or detailed guide) for any new functionality in this pull request: [y]
• I confirm that I have added a changelog entry to the changelog (we will add a link to this PR as part of the review): [y]
• I confirm that I have permission to release this code under the GPL3 license: [y]