Don't require non-const reference to call notify()

[scramsby]

This fixes notify() and variables_map::notify() to not require a non-const reference to the variables_map. This is useful if you are passing this object from one module to another and want to trigger handling of those variables in the new module's context without enabling that new module to mutate the shared map's state.

[codecov]

Codecov Report

Merging #79 into develop will increase coverage by 0.14%.
The diff coverage is 66.66%.

@@             Coverage Diff             @@
##           develop      #79      +/-   ##
===========================================
+ Coverage    49.89%   50.03%   +0.14%     
===========================================
  Files           23       23              
  Lines         1385     1387       +2     
  Branches       707      707              
===========================================
+ Hits           691      694       +3     
  Misses         110      110              
+ Partials       584      583       -1

Impacted Files	Coverage Δ
include/boost/program_options/variables_map.hpp	78.94% <ø> (+2.47%)	⬆️
src/variables_map.cpp	30.85% <66.66%> (ø)	⬆️
...de/boost/program_options/detail/value_semantic.hpp	43.75% <0%> (+1.56%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update d95d316...ecfef3d. Read the comment docs.