Add method to convert fit clearance numbers to plus/minus tols for evaluating assembly fitups

[lemon1324]

Computing a clearance fit using a tolerance loop will result in a positive number with distribution; some measn should be developed to allow conversion of this fit to:

1. unknown variation in linear position based on the value being from the clearance distribution and the offset in an unknown direction.

Variation in a known direction may benefit from a convenience function to allow more easy use of a clearance value in linear tolerance stacks.

[lemon1324]

Note there are two cases to implement here:

1. unknown direction and also unknown magnitude within the bounds
2. Unknown direction but position is at the bound (i.e. limit of motion)