Support warmstarts

[ericphanson]

The solvers accept an InitialPtFile to provide a starting point. This could be hooked up to MOI to allow the user to specify an initial search point (aka a warmstart). This probably wouldn't be too hard because we can reuse some of our printing code.

[ericphanson]

It would be cool also to have some way to load the problem using e.g. BigFloats, then write + solve it at 64 bit precision with plain SDPA, use that solution as an initial point, and then solve it again at high precision with SDPA-GMP. I wonder if that could end up a lot quicker than just using SDPA-GMP the whole way.

We could probably do it in a such a way that the problem data doesn't have to get loaded into the Optimizer object twice, and of course only do the presolve once as well.