Also added automatic detection of the right platform to the OpenMM tests.
This lets me catch errors on OpenCL, rather than missing them becuase
it was all reference or CPU

…ntext for

bond, angle and torsion forces

All the forces now use this, which has sped things up a lot

…er than

via a SireCAS::Expression

…so that it

is easier to perform just steps of dynamics without any extra overhead.

This is as fast as calling step on the underlying OpenMM integrator. It lets
us do lambda scans using the dynamics interface

[ci skip]

…all but Windows

… installed on the Windows GH Actions runners.

…rectly.

[ci skip]

…ux. Some minor changes, plus fixing the version of gcc to be the same as used on conda-forge

…g this to set the

compiler version on Linux, while hopefully leaving it alone for other OSs.

This does work - a local test shows that gemmi now passes when the compiler versions
match.

conda_build_config.yaml

[ci skip]