Create softcover_spine_calc_inches.py

[solanus1234]

Creates a new document with spine area defined for softcover books, based on size, paper weight, and number of pages. Includes a rotated text box in spine.

[aoloe]

hi thanks for the pull request!

i've put a few comments...
you can take them as a simple commentary or use them to modify your code so that it better fits for a broader usage.

if you want to change your code, you simply can do that on your computer and make a commit to the repository and branch you used for the pull request. the pull request will automatically be updated.

otherwise, i will accept the script as is and do the changes myself...

as you prefer!