Final release version 0.7.7

Changelog

Version 0.7.7, June 3, 2021

Per A Brodtkorb (27):
* Added cartopy and matplotlib to requirements.txt
* Updated appveyor.yml, setup.cfg and setup.py
* Updated .gitignore to ignore .pytest_cache
* Corrected failing doctests in objects.py
* Updated version in _installation.py
* Updated failing docstrings for python 2.7 in objects.py.
* Added '# doctest: SKIP' to all plt.show() in order to avoid the doctests hangs on the testserver.
* Fixed a bug in _info_functional.py
* Updated pycodestyle exlude section in setup.cfg Prettified _examples.py, examples_object_oriented.py and core.py
* Updated pycodestyle ignore section in setup.cfg
* Added doctest option to setup.cfg
* Removed print statements in test_objects.py
* Return "NotImplemented" instead of raising "NotImplementedError" in Nvector.mul and Nvector.div in objects.py
* Fixed .travis.yml so that he file paths in coverage.xml is discoverable
under the sonar.sources folder. The problem is that SonarQube is
analysing the checked-out source code (in src/nvector) but the actual
unit tests and coverage.py is run against the installed code (in
build/lib/nvector). Thus the absolute files paths to the installed code
in the generated coverage.xml were causing Sonar to show no coverage.
The workaround was to use sed in the pipeline to replace every path to
build/lib/nvector with src/nvector in coverage.xml.
* Fixed a bug: Identical expressions should not be used on both sides of a binary operator in test:objects.py.
* Updated solutions to example 9
* Added greatcircle method to GeoPoint.distance_and_azimuth in objects.py
* Added _base_angle function that makes sure an angle is between -pi and pi.
* Added test_direct_and_inverse in test_objects.py
* Added interp_nvectors to docs/reference/nvector_summary.rst
* Added vectorized interpolation routines: interp_nvectors function to core.py and Nvector.interpolate to objects.py.
* Put try except around code in use_docstring to avoid attribute 'doc'
of 'type' objects is not writable errors for python2.
* Added interp_nvectors
* Reorganized _displace_great_circle
* Added check that depths also are equal on in _on_ellipsoid_path and in _on_great_circle_path
* Refactored code from use_docstring_from function into the use_docstring
function in _common.py
* Simplified the adding of examples to the docstrings of functions and classes in core.py and objects.py.