Some miscellaneous changes

[chrisbrahms]

1. More flexible indexing into HDF5Outputs with up to two string indices (e.g. output["stats", "peakpower"]) to avoid loading the whole statistics Dict into memory.
2. Bugfix in Fields.optcomp_taylor for weird inputs (e.g. strongly structured pulses)
3. New function Processing.getφ to extract the spectral phase from a field.
4. Log total number of steps during propagation, not just how many we've repeated.