Add support for un-prefixable units
There are a lot of units which can not be implemented because adding them to the current set of units an prefixes would lead to conflicts. Here is a non exhaustive list of potential conflicts seed in f93d18d0 and 0a3997f4.
- Centi-day
c-d
conflicts with candela-cd
- Peta-year
P-a
conflicts with pascal-Pa
- Milli-inch
m-in
conflicts with minute-min
- Parsec
pc
conflicts with pico-speed_of_lightp-c
- Atto-unit
a-u
conflicts with astronomical unit-au
All these cases could be solved if it is possible to add un-prefixable units. None of the above prefixed units should be actually used.