Burau estimate for braid entropy

[jeanluct]

For my review paper, I wrote a little code (burauest.m) that returns the Burau estimate for the topological entropy. Might as well merge this into braidlab.

One option is to add a flag burau or burauest to braid.entropy. But since burauest returns some extra info (like the optimal t parameter), might be better to get its own function? Could also leave the option to plot the result if no return parameter is specified.

[jeanluct]

Wrote a better version of this in the project lawkram, called buraubound.

Actually, would be worth implementing an improved version in a paper by Biryukov. Test this.