MDAnalysis.analysis.density: Implement parallelization or mark as unparallelizable

[marinegor]

Given a newly implemented parallelization (https://docs.mdanalysis.org/dev/documentation_pages/analysis/parallelization.html), implement it for all classes in analysis.density module. You may use analysis.RMSD in develop as a reference.

You need to add:

• appropriate implementation of result aggregation for AnalysisBase subclasses in analysis.density module (or explicitly limit available backends if the classes aren't parallelizable)
• a boilerplate fixture(s) to testsuite/analysis/conftest.py, analogous with existing ones
• a client_... fixtures to all tests using in testsuite/MDAnalysisTests/analysis/test_density.py, and modify the way run() method is called