Fixed #1111 Add module for sliding dot product; Include pyfftw as (soft) dependency

[NimaSarajpoor]

See #1111

Pull Request Checklist

Below is a simple checklist but please do not hesitate to ask for assistance!

• Fork, clone, and checkout the newest version of the code
• Create a new branch
• Make necessary code changes
• Install black (i.e., python -m pip install black or conda install -c conda-forge black)
• Install flake8 (i.e., python -m pip install flake8 or conda install -c conda-forge flake8)
• Install pytest-cov (i.e., python -m pip install pytest-cov or conda install -c conda-forge pytest-cov)
• Run black --exclude=".*\.ipynb" --extend-exclude=".venv" --diff ./ in the root stumpy directory
• Run flake8 --extend-exclude=.venv ./ in the root stumpy directory
• Run ./setup.sh dev && ./test.sh in the root stumpy directory
• Reference a Github issue (and create one if one doesn't already exist)

[gitnotebooks]

Review these changes at https://app.gitnotebooks.com/stumpy-dev/stumpy/pull/1118

[NimaSarajpoor]

@seanlaw
As suggested in this comment, I will create a module for sliding dot product in this PR, and put the functions/classes there. I've started with changing the current "sliding dot product" functions in core.py to help us follow the changes more easily.

[seanlaw]

Your changes look good to me

[NimaSarajpoor]

I will create a module for sliding dot product in this PR,

seanlaw approved these changes

To make sure we are on the same page, I assume it is still okay to push changes here in this PR.

[seanlaw]

To make sure we are on the same page, I assume it is still okay to push changes here in this PR.

Yes, should be fine. If you feel like there are multiple distinct pieces or logical checkpoints then you may consider splitting it into multiple PRs. But if you are able to keep it simple, then we can do it here