Add vertical distance for vertical cover

[sh-nguyen]

Add option to specify the vertical distance from the workarea to the base of the fully extended vertical cover.

Description

Add new Window position variable that is used in calculating the position of the window.

Motivation and Context

This allows the workarea to be on a different plane to the bottom of the fully extended vertical cover.

How has this been tested?

Tested the formula change via simulation, and the integration via real world testing of two window shades that are different lengths mounted at the same height above the floor. Have confirmed that the bottom of the shades are level with each other throughout the day, when other parameters are kept the same.

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Enhancement (non-breaking which updates existing code)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Other

Checklist

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.

[carterbox]

You should add a longer description of this new parameter in the "data_description" section so that it appears in the HA GUI during configuration. Then users don't have to come to GitHub and scroll through the README to figure out the meaning of this new parameter.