Merge branch 'test_usercounters' of git://github.com/biojppm/benchmark into biojppm-test_usercounters

This prevents errors when additional non-timing data are present in
the JSON that is loaded, for example when complexity data has been
computed (see #379).

[ci-skip]

- Epsilon is now understood as relative to expected value.
- Improve error messages for epsilon checks.

This is needed for examining the values of user counters (needed
for #348). It is also needed for checking the values of standard
benchmark results like items_processed or complexities (for example,
checking the standard deviation is needed for unit testing #357
as discussed in #362).

The tests are still missing a way to check actual validity of
numerical results; this will be done next. As they currently are,
the tests pass, but the problem detected with #378 is still
standing and the results with non-standard counters are wrong.