Meaning and precision of variable names

[matthiasschaub]

Sometimes it is not very clear what the a variable means because of confusing naming and/or missing documentation.
See also discussion: #37 (comment)

Some examples:

current variable	proposed variable	(should) refer to
bpolys	bpolys	GeojJSON (Feature or FeatureCollection) in CLI/API
bpolys	feature	GeoJSON (Feature) in oqt.py and indicator/report classes
feature_id	fid / feature_id	unique value of a certain column
fid_field	fid_attrib / feature_id_attribute	certain column with unique values
layer / layer_name	layer_key	key of a layer
layer	layer	object of the class Layer
indicator / indicator_name	indicator_key	key of a indicator
indicator	indicator	object of the class indicator
report / report_name	report_key	key of a report
report	report	object of the class report

Also maybe rename 'layer' to 'topic' (see #406)

[matthiasschaub]

Linked to issue #9