Refactor charge neutrality Recipe

This merge requests refactors asr.charge_neutrality and adds new features. In particular, the 'behind the curtain' functionality of evaluating the chemical potential conditions from the monolayer host crystal will be outsourced into a detailed Recipe to evaluate the stable chemical potential conditions. In addition, frozen in defects as well as two different evaluation temperatures should be included. In the entire process, we should try to not break compatibility with the QPOD database.

Here's a few milestones and tasks that need to be finished before this merge request can be marked as ready:

• Implement two different temperatures: synthesis temperature T1, and evaluation temperature T2 (by default, those two temperatures should be equal and set to RT)
• Implement fixed concentrations
• Interface this Recipe with another Recipe solely focusing on the chemical potentials (@jiban_k)
• Once merge request !557 (merged) is merged into old-master, merge the old-master branch into this one (refactor-charge-neutrality)
• One chemical potential condition should lead to one results outcome
• Update tests
• Clear input argument structure
• Result object compatibility for QPOD

Edit: this merge request also contains a small style fix for asr.defectlinks that is not worth creating a separate merge request for as well as a web panel fix for asr.gs and defect systems (see #129 (closed)).