Rename the term Ticket Quality to Ticket Rank for better readability

[masih]

The Converge ticket selection mechanism previously used the term "quality" to determine which ticket is better where the lower the quality value the better the ticket.

Additionally, the term "quality" is already used in GPBFT for QUALITY phase, which is a totally different thing.

To avoid terminology confusion, and for a more intuitive notion in the context of ticket selection use the term "rank" instead, where instead one would say: whichever ticket ranks first is better, i.e. the smaller the value of rank the better the ticket.

While at it, update the docs on the compute function.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 79.44%. Comparing base (ff0b6f5) to head (795542e).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #633      +/-   ##
==========================================
- Coverage   79.83%   79.44%   -0.39%     
==========================================
  Files          52       52              
  Lines        4676     4676              
==========================================
- Hits         3733     3715      -18     
- Misses        592      601       +9     
- Partials      351      360       +9     

Files with missing lines	Coverage Δ
gpbft/gpbft.go	87.30% <100.00%> (ø)
gpbft/ticket_rank.go	84.61% <100.00%> (ø)

... and 4 files with indirect coverage changes

[Stebalien]

(I don't think this one will be controversial...)