added Random slump parameter for tempering

[IVinterbladh]

added Random slump parameter for tempering so the mpi threads do not get out of sync

Description

Please edit this text to include a summary of the change and which issue is fixed.

Checklist

• make test passes with no errors

• the source code is well documented

• new functionality includes unittests

• the user-manual has been updated to reflect the changes

• I have installed the provided commit hook to auto-format commits (requires clang-format):

  ./scripts/git-pre-commit-format install

• code naming scheme follows the convention:

  Style	Elements
  PascalCase	classes, namespaces
  camelCase	functions
  snake_case	variables

[mlund]

Thanks!