Suggestion: Add SI Units/Prefixes to units.dat

[fdt-dev]

In many cases it can be helpful to be able to add a SI prefix (for example mega or giga or yotta) to a unit. (Actually to be precise: multiply a SI prefix with the unit). My proposal therefore would be to add SI prefixes to the file units.dat as unitless numbers. Here's what that could look like:

[SI prefixes]
yotta              = 1000 zetta
zetta              = 1000 exa
exa                = 1000 peta
peta               = 1000 tera
tera               = 1000 giga
giga               = 1000 mega
mega               = 1000 kilo
kilo               = 1000 unit
hekto              = 100 unit
deca               = 10 unit
deci               = 0.1 unit
centi              = 0.01 unit
milli              = 0.001 unit
micro              = 0.001 milli
nano               = 0.001 micro
pico               = 0.001 nano
femto              = 0.001 pico
atto               = 0.001 femto
zepto              = 0.001 atto
yocto              = 0.001 zepto

Tested and working. For example in order to get the unit GeV (Giga Electronvolt) with the above entries in units.dat it is possible to enter "giga * ev" into the unit entry. ConvertAll correctly translates that to 1E+09 eV. Same for "pico * Farad" --> 1E-12 Farad.