String conversion in header-only mode

[thorstenhater]

Please consider making string conversion available in the header-only version
of units.

[phlptp]

the string parsing is a pretty big block of code, not well suited to a header only scenario. I could probably smash it all together into a single header, but that would make the compilation time pretty bad when used in multiple places.

[thorstenhater]

The background is that in terms of dependency management, especially transitive dependency, would
be substantially easier. Would a possible compromise be to gate this behind an extra flag, eg
UNITS_HEADER_ONLY_CONVERSIONS?

[phlptp]

Technically yes, just about anything is possible. Would you want the full capabilities of the string conversion methods, or would a very limited subset of the capabilities suffice?

[thorstenhater]

I am not entirely sure what that would entail, but my needs can be summarized as converting quantities (precise_measurement) of the form value unit from and to string. No flags beyond the default and unit
is at most as complicated as F/m^2.