Add min and max implementation for Cyclomatic

[giovannitangredi]

This PR adds minimum and maximum value for the Cyclomatic metric, adding some new unit testing to handle some corner cases and partly resolving Issue #410 .

[calixteman]

It would be nice (if it's possible) to add some references (papers, websites, ...) which could help to explain why min/max of cyclomatic is interesting. If they don't exist or if they aren't interesting, it isn't a problem, it's more out of curiosity (and I make some archeology on github I always appreciate to have some documentation (even if I don't do it myself all the time ;))).
Anyway we compute mean, so why not min/max, median, ... whatever I don't know.

[codecov-commenter]

Codecov Report

Merging #699 (cde25d8) into master (a507c7d) will increase coverage by 0.36%.
The diff coverage is 78.68%.

@@            Coverage Diff             @@
##           master     #699      +/-   ##
==========================================
+ Coverage   37.16%   37.52%   +0.36%     
==========================================
  Files          50       50              
  Lines        6294     6348      +54     
  Branches      936      940       +4     
==========================================
+ Hits         2339     2382      +43     
- Misses       3311     3318       +7     
- Partials      644      648       +4     

Impacted Files	Coverage Δ
rust-code-analysis-web/src/web/server.rs	60.54% <ø> (ø)
src/metrics/cyclomatic.rs	74.81% <76.78%> (+1.55%)	⬆️
src/spaces.rs	65.69% <100.00%> (+1.02%)	⬆️

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[giovannitangredi]

I have done the following changes:
Removed self_cyclomatic.

Added a new field called cyclomatic_sum that stores the sum of all cyclomatic for each space merged, similar to the old usage of cyclomatic. Now this value is used as sum and it is also used for calculate average. This field is merged when the merge funciton is called.

Now cyclomatic only stores the cyclomatic value of the single space/method, it doesn't get modified by the merge. This value is used when computing min and max and it is added to the sum when the parsing of the space is completed (implemented in the compute_minmax function of cyclomatic).

This changes have been done because when we merge 2 states, the first one could still be uncompleted and so the min and max cannot be computed correctly, now the min and max are only computed when a state has been completed.

[calixteman]

LGTM. Thank you for doing that.