Spreadsheet to owl conversion: match all columns to owl or rdfs properties

Following #5, this is an extension of the conversion of the [term spreadsheet](https://docs.google.com/spreadsheets/d/16pC2cDsdxlzv2CzlNMtStqt5-xHwDEsU6MjZVxWhrE4/edit?usp=sharing) into an owl file (limited to NIDM-Results). 

Each column in the [term spreadsheet](https://docs.google.com/spreadsheets/d/16pC2cDsdxlzv2CzlNMtStqt5-xHwDEsU6MjZVxWhrE4/edit?usp=sharing) was matched to a field in the ontology. When no corresponding property was found (e.g. "General Note") the text was copied as a comment.

@nicholsn: as I have not yet managed to install Protege on my machine, I have not updated the `owl` file yet (only the `turtle` version) but I hope I will be able to do it soon... The conversion code is in the [same ipython notebook](https://github.com/cmaumet/ni-o/blob/fromSpreadsheet/notebooks/NIDM-Results-Terms2RDF.ipynb), let me know what you think.




`nidm:stationaryRandomField` is now a class (and should start with an upper case cf. issue #10) with no domain as in commit f83d562.