Add Codecov Report in PR (new cookiecutting standard)

[bobleesj]

Please refer to diffpy/diffpy.stretched-nmf#133 - If Codespell (another issue created in this repository) has not been configured, it maybe easier to re-cookiecut the package. No README/license files have been modified.

[bobleesj]

Closed implemented

[bobleesj]

@cadenmyers13 seems like in the PR we have to write "Closes" for each issue like

Closes #X
Closes #X
Closes #X