add utils/include/edm4eic/unit_system.h

[veprbl]

Defines constants to support EDM4hep's unit system to be adopted as the default for EICrecon.

[c-dilks]

• length unit is mm := 1
• choose either full SI prefix, abbreviation, or both; e.g., millimeter vs. mm, or microsecond vs us
• some comments are incorrect, e.g. // mega electron volt on eV

[veprbl]

• length unit is mm := 1

Fixed

• choose either full SI prefix, abbreviation, or both; e.g., millimeter vs. mm, or microsecond vs us

I'm trying to chose abbreviation here. C++ doesn't support full unicode to use $\mu$. This is made minimal on purpose to avoid introducing alternative ways to do things.

• some comments are incorrect, e.g. // mega electron volt on eV

Fixed

[c-dilks]

Perhaps this would be better in EDM4hep, for everyone's benefit.

[veprbl]

Filed downstream PR eic/EICrecon#671 to start using this.

[wdconinc]

Only noticed this PR now. This looks good.