geodynamics/pylith: PyLith v2.2.1

Added new examples. examples/3d/subduction: New suite of examples for a 3-D subduction zone. This intermediate level suite of examples illustrates a wide range of PyLith features for quasi-static simulations. examples/2d/subduction: Added quasi-static spontaneous rupture earthquake cycle examples (Steps 5 and 6) for slip-weakening and rate- and state-friction. These new examples make use of ParaView Python scripts to facilitate using ParaView with PyLith. Improved the PyLith manual Added diagram to guide users on which installation method best meets their needs. Added instructions for how to use the Windows Subsystem for Linux to install the PyLith Linux binary on systems running Windows 10. Fixed bug in generating Xdmf files for 2-D vector output. Converted Xdmf generator from C++ to Python for more robust generation of Xdmf files from Python scripts. Updated spatialdata to v1.9.10. Improved error messages when reading SimpleDB and SimpleGridDB files. Updated PyLith parameter viewer to v1.1.0. Application and documentation are now available on line at https://geodynamics.github.io/pylith_parameters. Small fix to insure hierarchy path listed matches the one for PyLith. Updated PETSc to v3.7.6. See the PETSc documentation for a summary of all of the changes. Switched to using CentOS 6.9 for Linux binary builds to insure compatibility with glibc 2.12 and later