In order to work on the broadening in Abins, some adjustments are
being made to how these functions are called and it is made possible
to request different broadening schemes. A reimplementation of the
existing broadening is to be implemented as 'legacy' for ease of
comparison.

At this stage 'legacy' is the actual legacy implementation, while
'legacy_plus' is a similar implementation in the new framework. This
will replace the 'legacy' code as soon as possible.

The 'legacy' and 'legacy_plus' results give very similar results and
timing data, so while 'legacy_plus' does not provide full
compatibility it is a fair basis for comparison with new methods.