Store MadxUnits by composition in Model

Since the MadxUnits instance is `madx` specific (currently), it is
created in the frame (at the time the `madx` instance is created).
Rename public interface functions of util.units
Pass evaluate function to SymbolicExpression

Rather than a complete Madx object.
Load MAD-X units from config file
Merge branch 'units-config'

Resolves #46, #56