Flatten odml class hierarchy

[mpsonntag]

This PR flattens the odml class hierarchy since it is unlikely that we will support differing odML class implementations any time soon.

• removing base._baseobj class, making BaseObject the root odML class.
• removing doc.Document class, making BaseDocument the only odML document class.
• removing section.Section class, making BaseSection the only odML section class.
• removing property.Property class making BaseProperty the only odML property class.

The PR also

• reorders various magic methods to the top of their respective class to easier spot where custom magic methods are defined.
• renames baseobject and sectionable to BaseObject and Sectionable respectively.
• removes outdated odmlparser.is_valid_attribute method which had already previously copied to the new dictparser class.

[coveralls]

Coverage increased (+0.2%) to 74.627% when pulling f6dd160 on mpsonntag:slimFast into 23f9181 on G-Node:master.

[achilleas-k]

Well, this is absolutely lovely. Approved.