simplified usage of cookiecutter variables using derived variables

[jspaaks]

In this PR:

• simplified templating expressions by using a hook to change user input to their safe equivalents, e.g. MY python Project becomes my-python-project
• changed project_short_description to package_short_description
• changed project_slug to package_name
• changed open_source_license to just license
• Updated the table with explanations a bit
• got rid of using the package name for naming the module at the deepest level of nesting, renamed that file to my_module.py(i.e. without any templating)
• dashes are now allowed in the project name
• changed the order of questions: package first, project later

Refs: #152

Works except for the top level directory for some reason because cookiecutter first creates the top level directory, then runs the hooks/pre_gen_project.py hook. That means any updates you do to the project_name from the pre_gen_project hook come too late. This might be fixable by adding a post hook that renames the directory afterwards. But anyway, this behavior is not worse than what we had before.

To test (assuming you have cookiecutter already installed in user space):

cd $(mktemp -d --tmpdir python-template.XXXXXX)
git clone https://github.com/NLeSC/python-template
cd python-template
git checkout 152-derived-cookiecutter-properties
cd ..
cookiecutter python-template
# (accept defaults)

Here is the tree after running cookiecutter with the defaults:

my-python-project/
├── CHANGELOG.rst
├── CITATION.cff
├── CODE_OF_CONDUCT.rst
├── CONTRIBUTING.rst
├── docs
│   ├── conf.py
│   ├── index.rst
│   ├── make.bat
│   ├── Makefile
│   ├── _static
│   │   └── theme_overrides.css
│   └── _templates
├── LICENSE
├── MANIFEST.in
├── my_python_package
│   ├── __init__.py
│   ├── my_module.py
│   └── __version__.py
├── NOTICE
├── project_setup.md
├── README.rst
├── setup.cfg
├── setup.py
└── tests
    ├── __init__.py
    └── test_my_module.py

Now check the generated package:

cd my-python-project
# create a virtual environment and install all dependencies in it including dev deps
python3 -m venv env
source env/bin/activate
pip install --upgrade pip wheel setuptools
pip install .[dev]

# should pass and print a coverage report to the terminal (3 tests total)
pytest tests/

# test if you can build the documentation
cd docs
make html

[fdiblen]

I had a big relief after reviewing and approving this PR. 🍬

[jspaaks]

Thank you Faruk for the review and your code contributions!