LinkInstance wrappers and tests

[dariaivanciucova]

Purpose

This PR adds wrappers for the RevitLinkInstance class.

• Renamed LinkInstance to RevitLinkInstance
• Added Revit Nodes tests for RevitLinkInstance and LinkElement methods

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
• [x] The level of testing this PR includes is appropriate
• [x] User facing strings, if any, are extracted into *.resx files
• [x] Snapshot of UI changes, if any.

Reviewers

@Mikhinja @reddyashish

FYIs

@dnenov @Amoursol

[dnenov]

@Mikhinja - should we now merge this PR?

[dariaivanciucova]

hey @Mikhinja, is this still pending review?

[Mikhinja]

Sorry, I thought @dnenov had an additional change request, I'll merge it now