Return %syn tiers

For analysis it can be useful to also return the `%syn` tier after the `%mor` tier. The required information for this should be present in the Alpino parses, but it will require interpretation. A list of the `rel` attributes is already present in GrETEL and an explanation of the `%syn` tier can be found here: http://www.cs.cmu.edu/~dhdavis/guide8.pdf

@JanOdijk wrote a [script](https://github.com/UUDigitalHumanitieslab/AnnCor-scripts/files/1894101/lassy-ca.zip) last year to calculate the tuples from an Alpino parse and do comparisons between two trees. For this purpose only calculating those tuples should be sufficient.

(More explanation about this script can be found in the mail of Monday, April 9)