add hash table

[ac812]

Notes from Anton:

1. Some simple basics about data-structures - What’s the difference between a hash and an array, etc. In particular the overheads around using certain data structures over others.

We do a lot of dynamic programming in my practicals so while 1 hash table lookup might be fine in a regular script and make absolutely no difference, if you are traversing a matrix with millions of entries the overhead quickly builds up to the point where the code is running much much slower.

In my practicals, we managed to get some code running significantly faster by using integer lookups of an integer matrix instead of hash table lookups. But they seemed very unfamiliar with these things.