Adapting some of Prof. Scott's suggested changes.

[wensley-rushing]

This pull request updates some wording in the description of the opensees package, taking into consideration some of the changes Prof. Scott proposed here. Some changes include:

• Various wording changes suggested by Prof. Scott are adopted.
• The name of the opensees package is being changed to sees. This will avoid implying any preference between the standard OpenSees Python packages.
• The link to https://pypi.org/project/openseespy has been changed to point to the Python installation page of the official OpenSees documentation (ie, this). This page sounds like a good source of truth on the matter, and is expected to document all officially supported installation methods.

Please let me know any suggestions!