Herwig7_i: Add an option for the shower scale variations

Adding a function in the setup to enable shower scale variations, as done here. As requested in this JIRA, the scale variations need to be executed whenever the Matrix Element gets calculated by Herwig, this is now implemented here. The function can also be used from a JO. This MR is done as part of this JIRA.

Since I already update some stuff, I have corrected a check in the tune command whether a supported tune scheme is used.