Additional parameters for Spherical shell geometry and initial conditions 2

Updated geometry_model/spherical_shell.cc
Can specify number of cells in the initial, coarse mesh
   - additional comments and more descriptive variable names

New parameter:
   Cells along circumference
     - Default = 12 (Backwards compatible)

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Updated initial_conditions/spherical_shell.cc
Can specify the number of convective cells and their orientation
   - additional comments and more descriptive variable names
   - SphericalHexagonalPerturbation is backwards compatible

New parameters:
  Angular mode

  Rotation offset
Merge branch 'master' of https://github.com/WDurkin/aspect