Add support for specifying an initial mapping

[proteneer]

This PR adds support for pre-specifying an initial mapping (that can be generated using another cheaper method). This enforces the initial mapping to be exact (unlike specifying via priority_idxs which allows for UNMAPPED and skipping the node).

[badisa]

Been thinking about this PR a lot recently, seems super useful.

One point I had brought up online was that if the initial mapping is generated with different parameters (say allowing planar torsions to be mapped) the initial mapping won't be rejected even if it would fail to be generated using the new parameters. Don't think this is an issue, but does add a bit of nuance if we start using this heavily.