Determine HTTP query arguments for TPF requests automatically

The three arguments for providing the subject, the predicate, and the object of TPF requests are typically "subject", "predicate", and "object". However, this may not be the case for all TPF servers. Instead, TPF servers can chose what these arguments are and declare them in the [Hydra](http://www.markus-lanthaler.com/hydra/) description returned in each response to a TPF request.

Currently, HeFQUIN can use only the aforementioned typical argument names, which are hard-coded in `TPFInterfaceUtils`. We have to change that and extract the argument names from the Hydra description.

Start with TPFInterfaceUtils and trace how HeFQUIN builds TPF requests and receives each response. Inspect the returned Hydra description to determine where the subject, predicate, and object argument names are declared. Done means requests use those declared names instead of only the hard-coded defaults.

Written by the indexing model from the issue text.