Comment changes

[T-Nicholls]

Fully updated the comments on element.py to produce documentation in the desired manner.
Changed:

• Added a section listing the private attributes.
• Changed return types comments on multiple functions to be clearer.
• Made commenting syntax and grammar consistent throughout.
• Changed the description of cell to better reflect its position in the accelerator's hierarchy.
• Fixed error caused by trailing whitespace introduced by above changes.
• Split lines longer that 80 cols in two.
• Ensured conformity with standard controls group python style.
• Updated comments on functions to more accurately reflect their return types, all will now return true from isinstance().

[coveralls]

Coverage remained the same at 94.541% when pulling d56309e on T-Nicholls:commentChanges into 27c4c21 on willrogers:master.

[T-Nicholls]

• Fixed multiple autodocs warnings and style errors.
• Added new description to multiple, previously blank, methods.
• Changed grammar, formatting and wording throughout for better clarity.
• Ensured consistency between the autodocs display of all classes.
• Corrected all files to have a maximum line length of 80 cols.
• Removed unnecessary headings from sections where they do not apply to what is displayed.
• Corrected raises section on a few functions.

[T-Nicholls]

• Corrected return types for multiple functions.
• Corrected the raised error type of _raw_phys_to_eng of PchipUnitConv
• Corrected wording in the init args comments of multiple classes.
• Removed an incorrect description.
• Added inherited attributes to attributes sections of applicable classes.
• Fixed an issue with s position of the first element in the lattice having type int instead of float.
• Added a missing argument comment to set_value.
• Correctly capitalised PV in comments throughout all files.

[T-Nicholls]

I've made all the improvements/changes to pytac and it's documentation that I will for the moment and so this branch is ready to merge provided it is mistake free.