Allow setting an alternative root by tag for coverage graph

[choller]

This allows the graph to be adjusted by a new hash parameter "root" that looks for the specified name in the data set as a direct child of the original root (this does not support recursive lookups, but it is also unlikely that this would be needed).

Assuming that the report tag is the same as the report configuration name, this can then be used to automatically display IPC graphs correctly.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 69.99%. Comparing base (df19a65) to head (78c8516).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #976 +/- ## ======================================= Coverage 69.99% 69.99% ======================================= Files 150 150 Lines 9265 9265 Branches 45 45 ======================================= Hits 6485 6485 Misses 2759 2759 Partials 21 21 ```

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