ENH: Add convergence criteria and fix registration verbosity issue

[tbirdso]

Changes:

• Added convergence criteria for MeanSquaresRegistrar, DiffeoRegistrar, and PointSetEntropyRegistrar Python classes. Criteria may be set as function parameters.
• Added examples demonstrating convergence criteria to MeshToMeshRegistration.ipynb example notebook
• Fixed printout issue where an additional observer was added to the optimizer with each function call by making verbosity a member of MeshToMeshRegistrar and adding the observer in the initializer only
• Removed mesh write option at the end of registration to emphasize modularity. MeshToMeshRegistration.ipynb demonstrates use of itk.meshwrite to save registration results.
• Updated PointSetEntropyRegistrar pytest to rely on Euclidean distance metric (which consistently converges) rather than JHCT

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[tbirdso]

Rebased on #57.