Derive Dimension class from AtomicExpr instead of Dict.

Subclassing AtomicExpr is necessary for dimensional analysis.
Because I did not manage to inherit from both AtomicExpr and Dict
I have removed the last one and define all dict properties.
Add a function for (basic) dimensional analysis.
Function to simplify expressions with quantities and units.

Add also an example using this function to simplify the solution
to an equation.