Refactor the unit.py module to make it simplier and flexible

[o-murphy]

• make the opperations with raw values available
• add an arithmetic support between units of the same type and between different unit types|
• make the arithmetic operations to return stacked type of units if the operation returns unavailable unit in base

[dbookstaber]

Some notes on doing arithmetic on Units:

Example: We have a some_distance instance of Distance, and we want to double it.

Here is one kludge:
Distance(2*some_distance.raw_value, Distance.Inch) << some_distance.unit

Problem: We have to read through the Distance class to see that its raw_value is in units of Distance.Inch.

Solution: Give all AbstractUnits child classes a raw_units property (can be static) that returns the raw units for the subclass. For example, Distance.raw_units would return Distance.Inch. Then we could instead do:
Distance(2*some_distance.raw_value, some_distance.raw_units) << some_distance.unit

[o-murphy]

I played with that, I want to realize __add__, __mul__, __div__ etc for AbstractUnits, to use it directly with this syntax

d_instance = Unit.Inch(10)
double_distance = 2*d_instance

but simple math operations would be available just between 2 instances of same class, we'll not be able to multiply Distance to Velocity