UWGeodynamics v2.8

• Compatible with UW 2.8 (Python 3 only)
• Fix bugs reported on Github
• Update documentation on ReadTheDocs
• Support for SLCN (Semi Lagrangian Crank Nicholson method for Advection Diffusion)
  The default advection-diffusion remains SUPG but the user can switch to SLCN
  by setting:
  GEO.rcParams["advection.diffusion.method"] = "SLCN"
  I have added a simple example that illustrate the advection of the isotherm when
  a column of materials is extended (Thanks to P.Lovely).
• Add option to use pre-defined initial viscosities of the materials for the first solve.
  myMaterial.initial_viscosity = 1e22 * u.Pa * u.s
• Add option to pass the Lithostatic field as an initial stress condition.
  Note that the value used at the boundary is not updated as the conditions evolve during
  a simulation.
• Simplified Dockerfile / Docker Image
• UWGeodynamics Shape now behave as normal UW function.
  This means that classic UW function can be used to define a shape.
• IMPORTANT Shape Combination: The & operator now define the intersection of shapes, combining shapes is done
  using |. This is to be consistent with the previous point.
  The use of the + and - operator have also been removed as it was causing confusions.
• Passive Tracers now directly inherit from Swarm. There is no swarm method on the PassiveTracer
  object anymore. User can use it as a normal swarm.