Make PETSc and petsc4py required dependencies

[chrisrichardson]

Make PETSc and petsc4py both required dependencies in CMakeLists.txt

[jhale]

Fixed in #118

[chrisrichardson]

Actually, I think this was slightly wrong. petsc4py should not be a required dependency of the C++ build, only the Python build.

[jhale]

Looking at master, that is how it is.

[garth-wells]

The C++ build system should be fixed so it does not depend on petsc4py. All the necessary info is available via the PETSc headers.

[chrisrichardson]

-- Generating form files in demo and test and directories. May take some time...
-- ----------------------------------------------------------------------------------------
CMake Error at CMakeLists.txt:343 (message):
  Generation of form files failed:

  Traceback (most recent call last):

    File "/tmp/dolfinx/cpp/cmake/scripts/generate-form-files.py", line 21, in <module>
      from petsc4py import PETSc

  ModuleNotFoundError: No module named 'petsc4py'

[garth-wells]

We should get generate-form-files.py to take command lines arguments that are passed through to ffc.