Moved away from step terminology and fixed gait cycle stats terminology.

Fixes #109.

WalkingData > GaitData
WalkingData.steps > GaitData.gait_cycles
WalkingData.step_data > GaitData.gait_cycle_stats
WalkingData.plot_steps > GaitData.plot_gait_cycles

The keys in the WalkingData.step_data/GaitData.gait_cycle_stats where changed:

Cadence > Stride Frequency
Step Duration > Stride Duration
Step Length > Stride Length