Refactor inference time measurement through common interface

[PGijsbers]

PR #532 introduced the option to measure inference time for many frameworks, but it introduced a lot of code duplication along the way. The PR should be "finished" by:

• introducing a common interface that abstracts away a lot of the duplicated boilerplate code
• extend inference time measurement support to frameworks that don't have it yet
• add documentation

Another open issue (that may be tackled independently altogether), is that the inference time measurement can take a considerable amount of time. This may lead to the benchmark aborting a job because of the overhead, while it would otherwise would have been successful. There should be the option to do "best effort" inference time measurement within the time limit, or perhaps configure it dynamically. I don't have a concrete idea on how to do this robustly.