Updated instructions to create version tag.

Example for tag "v2.7".

1. Ensure that Python files are formatted following convention::

1. Ensure that all changes in source files are committed.

2. Update file formatting following conventions.

   a. For Python files::

        make black

2. Update the history sections of source files, commit changes.

   b. Eventually also cleanup Fortran sources.

   c. Commit these layout changes.

2. Update change logs:

3. Update the `CHANGELOG <../../../CHANGELOG>`.

   a. Update the `CHANGELOG <../../../CHANGELOG>`_.

4. Update the source file `doc/source/history.rst <../../source/history.rst>`_ of this chapter.

   b. Update the source file `doc/source/history.rst <../../source/history.rst>`_ of this chapter.

5. Change the release tag that will be shown in the documentation in the file

   c. Change the release tag that will be shown in the documentation in the file

      `doc/source/conf.py <../../source/conf.py>`_::

        # The full version, including alpha/beta/rc tags

        release = "v2.7"

6. Commit these changes::

   d. Commit and push these changes::

      git commit -m 'Tagged v2.7' doc/source/history.rst doc/source/conf.py CHANGELOG

       git commit -m 'Defined v2.7' .

       git push

7. Tag and push the code to the git repository::

3. Tag and push the code to the git repository::

      git tag -a 'v2.7' -m 'Tagged v2.7'

      git push origin v2.7