kwargs for get methods, added containing_index & param for Section

Section get methods accept containing_index (return sections containing an index) and param (for aligning index and within_slice) kwargs. Changed to passing kwargs between get methods to reduce duplication of code and docs.
Fixed node saving and loading with compression
Gross Weight Smoothed simplified to fix data spike

Rather than using coreg to extrapolate the offset is based on the remaining fuel to burn.
Tidy up in derived_parameters.py
Documentation improvement for coreg.
Fixed containing_index alignment in Section._get_condition and tests
Retained Gross Weight Smoothed units change to kgs
Merge branch 'master' of https://github.com/FlightDataServices/FlightDataAnalyzer

Conflicts:
	analysis_engine/derived_parameters.py
Gross Weight Smoothed fix and tests

Fixed tests and added real test data for Gross Weight Smoothed.
Merge branch 'master' of https://github.com/FlightDataServices/FlightDataAnalyzer