Integration.py: Complete reworking of how timesteps are calculated. 
Integration.py: Complete reworking of how timesteps are calculated. 

There are two principles at work here:
1) The timestep choice should respect the symmetry of the diffusion equation
   upon redefining Nref. For example, if we divide Nref by 2, theta decreases
   by 2, T increases by 2, and nu increases by 2. The end result is an
   identical diffusion, and we should respect that.
2) Extrapolation works much better when the relevant integrations are all use
   the same timesteps. Thus, we *do not* want to set the timestep based on dx.
   This may make things a little more confusing for users, but the gain in 
   stability is, I think, worth it.



git-svn-id: 4c7b13231a96299fde701bb5dec4bd2aaf383fc6@219 979d6bd5-6d4d-0410-bece-f567c23bd345

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