Fix issues #49 and #50

[lohedges]

This pull request closes #49 and closes #50.

For #49 we now add a small bias when performing the triclinic lattice reduction to avoid issues with numerical precision due to the fixed-width format of box lattice vectors and angles in molecular input files. I've added a test to confirm that the resulting box is stable, i.e. it no longer oscillates about two sets of angles. The box is stable regardless of the way in which the triclinic box is instantiated, i.e. from box vectors, or magnitudes and angles.

For #50 I've added an appropriately pinned run_constrained entry for rdkit, so that a compatible version of rdkit will be installed alongside Sire, which currently isn't the case. This is currently entered directly in the conda recipe. It may be desirable to add this to a requirements file to be consistent with the other entries, or if we have other optional requirements that also need constraining. (That said, I'm not sure how the logic would be expressed.)

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