Section: expose properties as .props
[test] TestSection: modify to use .props
Section: simplify, i.e. dict like, value access

Provides the value of proprties for Section['prop'] instead
of the Property. Should make it easier to work with odML

Cf issue #102
[test] Section: tests for dict like value access
Merge pull request #103 from gicmo/section_value_access

Section: more dict like access for values