Added Revit Units Properties

Purpose

REDO of PR. Previous PR was based on 2.16. This is a redo to current master branch.

To be used in conjunction with https://github.com/DynamoDS/Dynamo/pull/13895! This PR creates an entry inside Dynamo's PreferenceSettings which track the current document's units inside Revit. This is used to help scale background graphics relative to the current project settings thus making the background helpers inside Dynamo more useful.

this PR works with https://github.com/DynamoDS/Dynamo/pull/13895 PR on Dynamo side
it introduces a way for Dynamo Revit to set revit units inside the PreferenceSettings in Dynamo
to be used against the correct Dynamo Core (DynamoCore.dll can be manually referenced to make it work until release)
units will only be set once per Dynamo start - every time Dynamo is launched, the current document units will be set

generic 'Imperial' and 'Metric' Revit Units

imeprial metric units

setting Revit units to scale Dynamo grids

Declarations

Check these if you believe they are true

[x] The code base is in a better state after this PR
[x] Is documented according to the standards
[ ] The level of testing this PR includes is appropriate
[ ] User facing strings, if any, are extracted into *.resx files
[x] Snapshot of UI changes, if any.

Reviewers

@reddyashish @Amoursol

DynamoDS / DynamoRevit