figures/salt/fit-valley*: Use Asymptote images instead of PNGs

figures/salt/fit-valley*: Use Asymptote images instead of PNGs

Also upgrade to the 50x50 grid sawsim simulation, instead of the
previous 10x10 PNGs.  Basic command used to generate the histogram
fits:

  PYTHONPATH=. sawsim_hist_scan.py -f "-s cantilever,hooke,0.08 -N1 -s 'folded,null' -N8 -s 'unfolded,wlc,{0.4e-9,28.1e-9}' -k 'folded,unfolded,bell,{%g,%g}' -q folded" -r '[1e-3,1,30],[0.1e-9,0.3e-9,30]' --logx path/to/histogram