Enable Visibility Analysis component to report the percent of points seen within max_dist_

[chriswmackey]

This shouldn't be terribly difficult to add. It should be exposed an an option on the component inputs, though, since I think it doesn't feel like the desired default behavior.

To enable this, you'll have to first figure out the number of _view_points that are within a given distance of a particular grid point. Once you have that, it's pretty easy. You'll just have to edit this line of code inside the component:

https://github.com/ladybug-tools/ladybug-grasshopper/blob/master/ladybug_grasshopper/src/LB%20Visibility%20Percent.py#L147

Essentially, you'll want to divide by the number of points within the distance rather than the vec_count (aka. the total number of points). You'll also probably need something to catch the case that no points are within the max_dist_ since you'll otherwise get a ZeroDivisionError.

More information is here:
https://discourse.ladybug.tools/t/view-analysis-view-distance-and-direction-to-a-particular-building-facade/17778/2