Perform 3+1 split

[eschnett]

Performing a 3+1 split manually is often tedious but straightforward. Do this semi-automatically. It may not work well for the Einstein equations, where the step all the way to BSSN is very large, for this would be useful for many simpler wave-type equations that are interesting e.g. for cosmology.

[ianhinder]

This is beyond the scope of Kranc I think. Advanced symbolic calculations like this should be performed in xAct, which supports 3+1 decompositions natively. Kranc has initial support (#26) for using xAct expressions, though this is not yet complete.