Add possibility to query solving statistics

[hedtke]

fix #21

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 97.53%. Comparing base (a946921) to head (654894d).

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #22      +/-   ##
==========================================
+ Coverage   97.46%   97.53%   +0.06%     
==========================================
  Files           8        9       +1     
  Lines         158      162       +4     
  Branches       39       39              
==========================================
+ Hits          154      158       +4     
  Misses          3        3              
  Partials        1        1              

Files	Coverage Δ
include/scippp/model.hpp	96.15% <100.00%> (+0.32%)	:arrow_up:
include/scippp/statistics.hpp	100.00% <100.00%> (ø)