Sundry interface tweaks for 0.5

[ddahlbom]

• ωs -> available_energies
• all_exact_wave_vectors -> available_wave_vectors
• FeI2 classical tutorial now presents single-energy cuts without skew.
• No longer possible to define an intensity_formula without specifying a mode
• Default contraction mode is now :trace

I couldn't think of a nice way of making of forcing a user to specify a mode with every intensity call because of potential redundancy (or conflict) with an explicitly given formula. Put another way, the mode is really set in the formula. So, if we wanted to make sure all calls to an intensity function specify a mode explicitly, we would really have to require a formula (rather than a mode). I kind of like that idea, but it probably would make things too cumbersome for first time users. Let me know your thoughts if you don't like this arrangement.

Default :trace feels more natural to me (isn't overly biased toward people working with scattering data), but I don't think it's a hill I'm willing to die on.

@Lazersmoke You'll want to check that I've used the binning code in a reasonable way in the tutorial and make sure I haven't messed anything up when playing with the defaults.

[ddahlbom]

Ok, a formula is now required for all intensities functions.

I think I am happy with the current state of this. Let me know if there are any final objections.