Observation and Slice parsing uses now generators

Observation and Slice parsing as persistence has been rewritten to
use generators and reduce the memory consumption.
Remove unused code from the parser
Parse indicators using generators

Indicators are now parsed used generators. This, along with other
minor optimizations in the code should reduce the amount of used
memory.
Close #53

Generators are now used from the Parser to the Services layer. This
should result in a reduction in the memory used by the Receiver.
Merge branch 'enhancement-#53' into develop