flexible head direction

[mehulrastogi]

solves #70

[TomGeorge1234]

brilliant, I made a couple of minor tweaks; some warnings plus I think the head direction vector should be updated with the normalised velocity vector rather than straight-up velocity so its not biased in favour of faster speeds, thats all.

great job @mehulrastogi, thanks for a speedy update!

[mehulrastogi]

@TomGeorge1234 there is a typo in the last commit. Instead of warning.warn it should be warnings.warn

We should really set up the automatic tests. Want me to look into it?

[TomGeorge1234]

fixed. but you're right, this shouldn't happen, tests would be great actually, if you had time I'd really appreciate it!