D'oh

Fairly basic, could obviously be refactored but they are there to be exemplars
to give folks an idea of where they can start with the pipeline.

Adds config to isualisation and JSON producers

_keep_num() was incorrectly detecting overlaps

Identified when checking coverage while only testing utils

Found to be useful when writing tests

Otherwise features tend to cluster

As with VAFE, update LoudFE to expand to min duration when keeping n features

see: 72fc6b563b

The problem that I had overlooked until now (!!) due to only using small
exemplar videos for testing was that VideoActivityFeatureExtractor keeps
a fraction of small (0.5s) Features. This is not so problematic for a short
exemplar video, but ends up witha  lot of jumpy Features in an actual source
video.

Fix approach:

Like LoudnessFE, keep a specific number and expand the duration (which I think I
will do for LoudnessFE too), dropping any that would be consumed in that range

add LoudAudioFeatureExtractor._num_features (set in init, default: 5)

add _keep_num -- takes list of loudnesses and keeps top N (default: 5)

Add test cases that require reducing the number of Features

Facilitates unit tests of TTA

Pull out a one-liner for test scenarios to facilitate testing adjust()

Cases:

 - no features
 - features under target duration
 - features equal to target duration

Test _determine_margin() with margins specified as percentage

Tries to drop the lowest-scoring Features until the target time (range) is
reached. This is not optimised and a relatively naïve approach- there are many
inputs which would result in a non-ideal pruning.

TargetTimeAdjuster will adjust a list of Features until it is within an optional
margin of a target total duration.

Helper functions:

 - _determine_margin() :: figure out the max and min cutoff times, considering
   margin and margin strategy (percent / absolute)
 - _features_total_time() :: basic sum of list of Features' durations

TODO: rename to TargetDurationAdjuster ? rename 'strategy' ??

Adjusters will be used to modify a list of Features. This could either be:

 - to modify the overall set (eg to target a time)
 - to modify individual Features

The most important Adjuster will be one that targets an overall time, eg:

"modify this list of Features such that their times add up to 1 minute (either ±
a % or a hard limit)"

@see: feature_extractors.py::FeatureExtractor

Also rename positive test to indicate that