Low energy hadronic interactions: UrQMD interface

implement interface to UrQMD

according to Ralph, this is the easiest of the available LE models to work with

added Feature request label

mentioned in issue #162 (closed)

created merge request !100 (merged) to address this issue

mentioned in merge request !100 (merged)

changed milestone to %ICRC2019

I moved this one to ICRC2019, since it is not essential for the hadron-core milestone.

closed via merge request !100 (merged)

mentioned in commit 6f2ab230

mentioned in issue #424