Harmonize attribute naming with nixpy

[mpsonntag]

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

The create_section method is added to Document and Section and the create_property method to Section. The props alias attribute of properties is added to Section to close #299.

Let me know if I missed any nixpy attribute that should be ported to odml as well and whether we should unify __repr__ of Section and Property as well, since they are currently different.

See also #308 for completeness of this topic.

[coveralls]

Coverage increased (+0.1%) to 77.959% when pulling 7ac9973 on mpsonntag:funcharm into 4ce2e1b on G-Node:master.

[achilleas-k]

In addition to making __repr__ the same with nixpy which you brought up, we could also add the pprint pretty print method to odML.

[jgrewe]

lgtmt