docs for LinearDensity missing

[orbeckst]

The actual class docs for LinearDensity at http://pythonhosted.org/MDAnalysis/documentation_pages/analysis/lineardensity.html are missing.

[richardjgowers]

The docs exist, I think it's just missing .. automodule::, and maybe :members:

http://www.sphinx-doc.org/en/stable/ext/autodoc.html

Should just be a case of copying what has worked in a neighbouring module