Minor code format fix in __Regenerate.tt.
Fixed tabs in MultipleOutputHelper.ttinclude.
Changed MultipleOutputHelper.ttinclude to nest generated items under template (again).
Fixed tabs in Shared.ttinclude.
Merged UnitClasses.ttinclude and UnitClass.ttinclude to UnitClasses.tt.
Extracted UnitConverter.tt from __Regenerate.tt.

Removed old items:
Deleted    Src/UnitsNet/Generated Code/Includes/UnitConverter.ttinclude
Deleted    Src/UnitsNet/Generated Code/__Regenerate.tt
Deleted    Src/UnitsNet/Generated Code/__Regenerate.txt
Regenerated UnitTestClasses.tt. Generated items now nested under template.
Removed output of UnitTestClasses.tt.
Removed UnitTestClasses.txt.
Renamed folders Custom Code => CustomCode, Generated Code => GeneratedCode.
Removed missing items in UnitsNet.Tests.net35.csproj.
Added microgram and nanogram mass units.