Mute build warning for equality test on itself in Length2dTests.
Revert MultipleOutputHelper.ttinclude to original from nuget.

* Move back to project root.
* Revert modifications (adding the using() pattern)
* Update usages
Use T4MultiFile nuget in test project.

* Add nuget
* Use nuget's .ttinclude file
* Minor clean up
Add FromValueAndUnit() and In() tests.
Delete obsolete tests.

Covered in previous commit.
Remove TryConvert(), rename Convert() to As()

These were originally copied from UnitConverter class. Does not make sense to TryConvert now that the big unit enum was divided into a separate enum for each unit class. Convert() also did not make much sense, as you don't really convert Length to meters, but rather get that length in a meter representation.

* Update tests
Merge UnitTestClass.ttinclude into UnitTestClasses.tt.