Renamed SiDistance to Length.
Renamed SiDistance2d.cs to Position2D.cs
Renamed SiForce.cs to Force.cs.
Fixed comment.
Renamed SiMass to Mass.
Renamed SiPressure.cs to Pressure.
Renamed SiTorque.cs to Torque.cs
Renamed SiValue.cs to UnitValue.cs.
Renamed SiUnitsHelper.cs to UnitsHelper.cs
Renamed SiUnit to Unit.
Renamed SiUnitSystem.cs to UnitSystem.cs.
Renamed SiVoltage.cs to ElectricPotential.cs
Renamed Distance2dTests.cs to Position2DTests.cs
Renamed DistanceTests.cs to LengthTests.cs