Class able to deal with scientific units and make the appropriate
conversions, according to the
"Adopted Standards for Astronomical Catalogues"
The recursive definition is as follows:
Full_Unit = factor Complex_unit
Complex_Unit = single_Unit
| single_Unit OP single_Unit
| "log[" single_Unit OP single_Unit "]"
| "mag[" single_Unit OP single_Unit "]"
single_Unit = extended_UnitSymbol
| extended_UnitSymbol power
extended_UnitSymbol = UnitSymbol
| Magnitude_Prefix UnitSymbol
| "(" Full_Unit ")"
power = Number
OP = . | /
Basically, a unit consists in a value associated to a
unit symbol which is itself related to the SI