[RDF] 'Bag' to 'Seq'

[mpsonntag]

Currently the RDF class Bag is used to store odml.Property values. To note, that the order of the values is important, this PR switches Bag to the RDF container class Seq which implies order. Since rdflib currently does not support proper Seq behaviour with RDF li items, for now the index of the value items will be manually written as RDF properties, which rdflib supports when reading the file in again.

Any suggestions to solve this issue differently are welcome.

This PR further

• adds python versions to the CI builds.
• updates yaml testfiles to partially address Using pyyaml > 3.12 breaks tests #291.

[coveralls]

Coverage decreased (-0.02%) to 81.763% when pulling 4e4eeb7 on mpsonntag:bagToSeq into eb97b7f on G-Node:master.

[achilleas-k]

Pending Travis build fix