Improve documentation for unitsystem module.
Fix a bug when creating unit systems with some units of equal dim

The sort_units static method was keeping only one unit of each
dimension from the list given in arguments. This method was used
to ensure that base units were sorted in the same order as their
dimensions in the dimension system.
Ensure that there no duplicate dimensions in DimensionSystem

Until now it was possible to have several identical dimensions,
which has no sense.
Function to create prefixed units from a given units

Apply this ufnction in MKS and natural system.
Add MKSA system.