Create fix-param slider when stepsize cannot be given

[aseeland]

The current vue-slider allows only a constant interval between numbers.
However there are a lot of cases where at the beginning or end of the slider the stepsize does not match this constant. In this cases custom marks should be possible, and a list with all values. This list is translated to a fix-param with options.

For that, the spec has to be adjusted (in the Websocket-API it is already included).
Upstream docu: https://nightcatsama.github.io/vue-slider-component/#/basics/marks

See also VirtualProgrammingLab/documentation#60