Replace references to renamed master branch.

.github/create_release_from_gitlab_ci.sh

.gitlab-ci.yml

CONTRIBUTING.rst

LICENSE

README.rst

docs/algorithm_descriptions.rst

docs/installation.rst

docs/tutorial.rst

docs/usage.rst

enpt/__init__.py

enpt/cli.py

enpt/execution/__init__.py

enpt/execution/controller.py

enpt/io/__init__.py

enpt/io/reader.py

enpt/model/__init__.py

enpt/model/images/__init__.py

enpt/model/images/image_baseclasses.py

enpt/model/images/images_mapgeo.py

enpt/model/images/images_sensorgeo.py