prescribe to only use top level imports in MDAnalysis.analysis

[orbeckst]

The discussion on #577 resulted in the proposal to amend the Style Guide on Importing Modules to say

1. In MDAnalysis.analysis, all imports must be at the top level.
2. No analysis module is imported automatically at the MDAnalysis.analysis level to avoid breaking the installation when optional dependencies are missing.

This applies to new code. Old code that is not compliant should be refactored (raise an issue).

(Note: We already do 2 but it's really important so I am saying it here again.)

[orbeckst]

(Aw, I snatched issue #666)

[orbeckst]

Changed the rules for module imports in the Style Guide (version 27) under Module imports in MDanalysis.analysis.

Also added some formatting and sectioning and expanded the section on imports with key points from recent discussions (eg on #708 and #719 ).