removed yeart variable

- longitudinal analysis of variation
- cross-sectional analysis of variation for 2012
- some distributional charts

names were over-written

10%, 50%, 100% (not a sample!!)

explained sub-sample selection process

checked for transitions between different ‘valid observation’ states

added do_graphs switch
checks for valid gas values of 0 due to rounding of 100 < gcons < 250
to ‘nearest’ 500 - i.e 0!