Tidy up this repo

[kbuzzard]

Tidy up the project. Ideas:

1. Imperial subdirectory with dirs M1F, M2PM2 etc where we prove theorems in current courses.
2. Library xenalib, I want to put e.g. Keji's det(A)det(B) code and all the other stuff that everyone did that is not actually in mathlib in there. Chris' exp and cos etc. You PR it, you maintain it, if it breaks, never mind, hopefully someone will fix it. When stuff appears in mathlib it can be marked "xena library free".
3. My random public stuff e.g. lean and other files associated to blog posts, code written by me to do random stuff.

[kbuzzard]

4. Problem bank directory, with two subdirectories M1F and what_is_mathematics. M1F subdirectory contains numbered lists of problems, with Lean rating. What is mathematics directory contains the basic problems I will use to teach; they will mostly be original to me and will be questions which teach students first how to do propositions and logic, then all the other chapters I've written or half-written.

[kbuzzard]

5. html directory of bits of book which are actually finished? Should do basic title and front page stuff (or at least an html readme generated by sphinx) before I can even start on this. TODO