feat: imlpement generic Adjuster class

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

pipeline/adjusters.py

@@ -0,0 +1,25 @@
+"""adjusters.py -- adjust the gathered Features
+
+This is usually done to either modify to reduce the Features in some way.
+
+For example:
+
+ - TargetTimeAdjuster: drop Features until the target time is reached
+ - FeatureCountAdjuster: drop Features until the target number of Features is reached
+
+TODO: Consider eg a generic PredicateAdjuster -- supply a predicate/lambda that will be used to determine whether to keep a Feature or not.
+"""
+
+class Adjuster():
+    """Generic Adjuster class. Expects a list of Features and returns a list of Features."""
+
+    def __init__(self, features: list=[]):
+        """Initialize the Adjuster with Features.
+
+        NOTE: an empty feature list is permitted, since a FeatureExtractor may not produce features. Adjusters subclassing should be aware of this.
+        """
+        self.features = features
+
+
+    def adjust(self):
+        """Adjust the Features. Override this method in subclasses."""