Condense installation

Ok I've managed to condense things down.

Here are the steps to install g4sim and geant4:

```
> git clone git@github.com:alcap-org/g4sim.git
> cd g4sim
> ./install-geant4.sh
```

To compile g4sim:
```
> source env.sh
> cd cmake
> cmake .
> make
```

To run g4sim:
```
> cd ../alcap
> g4sim macros/test_run.mac
```

This should then produce a visualisation of the experiment and produce tracks for one photon from the target.

I've tested this on SLC5 here at UCL and will test on Merlin. If others could check that they can get it to work as well and post any issues they have then that would be great.

I will merge this branch in a couple of days if there are no major problems.