Tidy up powerpoint presentation

Claire

Minimal changes: increased size of images in slides 1, 10, 12, 13, 15, 18, 20 put figures into context in slide 3 (assuming CO2 quoted in kg) add BitCoin logo to slide 5 rewrite text in slide 12, 17 shortened text in slide 14, 16 better-res pics in slide 17 jpegs of just the three slides with significant changes uploaded