`diffkemp-out.yaml`: Add missing definitions for functions when comparing `sysctl` parameters

[PLukas2018]

Currently, when comparing sysctl parameters, the diffkemp-out.yaml does not contain definitions of all functions. We use result.graph for the extraction of the definition, and the variable contains only functions for the last compared group (sysctl parameter), because we clean the graph after each group comparison: https://github.com/diffkemp/diffkemp/blob/dce870dde6a78a691ae77dbdd9b6c871c698db08/diffkemp/compare.py#L46
This problem should potentially be solved by solving #403 (by creating the instance of the Result class for the groups / sysctl parameters).