Length instead of turns as an option

[ddivins]

Thank you. I really like this library.

I think it would be great if we could have an option to use turns or a height/length for the thread creation (this doesn't exist as far as I can tell). If using height/length you can cross reference the specs and determine how many turns are needed to hit the desired length.

If I want to make an M5x20 Bolt I need to do math. I am just starting to play with the spec options to add as a function like get_length_from_turns(designator, length) calls spec and does the math. Have not dived too deep..yet.