Decibel litteral

It would be really handy to write values like:

`units::dimensionless::dB_t Attenuation = -20_dB;`

However, dB is already defined as ... a decibyte automatically by the macro defining a byte and its multiples.

I can avoid this by disabling predefined data units , and add in my code:

`UNIT_ADD_LITERALS(dimensionless, dB, dB)`

Could we have this natively in the library?