Sherpa-like calling convention

[DougBurke]

Sherpa doesn't send in the n+1 energy grid get n data points, instead we send in elo,ehi arrays that can contain gaps. This is handled in the C++ interface code. It doesn't really make sense to have this here, but without it I'm not sure we can use it from Sherpa (should do some timing, also I've long wanted to change the Sherpa model evaluation code path which would be related)