nixpy style values and printing

[mpsonntag]

This PR aims to make working with odml and the odml part of nixpy more similar and closes #308 and #309.

Updates in printing Sections and Properties

• __repr__ of both Section and Property are now similar to the nixpy style.
• Section.__repr__ also features the immediate child Property count next to the child Section count.
• The nixpy style pprint methods have been added to both Section and Property.

Adding of 'Property.values' and deprecation of 'Property.value'

To make working with odML and nixpy more similar, the Property.value attribute has been marked deprecated and the Property.values attribute has been added. This is a significant change, since Property attributes are serialized to and from a file when saving / loading to any of the supported file formats.

Since I did not want any changes in the file format itself, Property.values is now mapped via the Property format to value on saving and back on loading similar to the handling of Property.dependency_value or Property.oid.

All the tests and documentation have been checked and updated, since this is quite a significant change and touches the Property format and all parsers.

[coveralls]

Coverage decreased (-0.8%) to 77.232% when pulling 3c0208c on mpsonntag:nixvalues into 25330df on G-Node:master.

[jgrewe]

lgtm,
the only thing I was wondering about is the e.g "section[0/5]" notation. On first glance it looked to me as if this indicates section number 0 of 5... which does not make sense.

I do not have a better idea, though (maybe the pipe?)