feat(c3) : Add ability to evaluate cost of C3 plan

[ebianchi]

Addresses #31

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This change is 

[github-actions]

LCOV of commit df22ca5 during C3 Coverage #92

Summary coverage rate:
  lines......: 92.8% (1644 of 1772 lines)
  functions..: 78.0% (135 of 173 functions)
  branches...: 52.6% (1280 of 2434 branches)

Files changed coverage rate:
                                            |Lines       |Functions  |Branches    
  Filename                                  |Rate     Num|Rate    Num|Rate     Num
  ================================================================================
  core/c3.cc                                | 8.0%    276|2171%    21|    -      0
  core/c3.h                                 |18.2%     22|1400%     3|    -      0
  core/lcs.cc                               |13.7%     51|1167%     6|    -      0
  core/lcs.h                                |17.6%     17|2667%     3|    -      0

[Meow404]

The overall structure really looks good to me 🔥 . I haven't taken a deep dive, but having some nested namespaces for trajectory evaluation would make sure they are well scoped. Letting the functions be declared static as part of a class is also an option but not required.