Make the unit standard value functions constexpr

[shossjer]

This change requires c++14 to compile but the code can if needed be changed to support c++11 by writing something like

    return unit == OpenRAVE::LU_Meter ? 1.0 :
           unit == OpenRAVE::LU_Millimeter ? 1e3 :
           ...
           throw ...;

[rdiankov]

Thanks~

[cielavenir]

don't worry, after Debian11 migration, our standard is C++14.

[cielavenir]

for record: this was reverted by https://github.com/rdiankov/openrave/commit/3bc6152ed7a26925c0bfc34d5bf40ac751617925