Add some simple tests for VectorOps::angle_diff
Fix max magnetisation angle difference

* And add an error check for large angle differences
Use symlinked mesh directory in each driver folder

Can't think of a better way around this stupid problem...
Update bem field test to use my semi implicit problem class

* So results are more relevant
* Required a few other changes
* Explain "spin angle" stuff
* use oomph_info not std::cout