Chemical potential should not stored as a config variable

[timcallow]

The chemical potential (config.mu) is really a property of the staticKS.Orbitals class and thus should be moved there. Storing it as a variable in config causes a bug when post-processing a set of orbitals under different conditions, because the occupation numbers are recalculated with the last saved value of config.mu instead of their actual chemical potential.

[timcallow]

This became a problem with PR #123, as the occupation numbers are automatically recomputed with this PR. Needs to be fixed before #123 can be merged.

[timcallow]

The issue is no longer a problem for #123 as the code has been reverted back to its old state of only computing occupation numbers when explicitly required. However in the long term the chemical potential should become a property of the Orbitals so keeping this issue open