document meanings of columns in verbose output

[dimpase]

currently with -g 1/2 -v 2 one sees

deg     sel   pairs        mat          density            new data         time(rd) in sec (real|cpu)
------------------------------------------------------------------------------------------------------

in the header, and while some of it can be guessed, some are totally cryptic. This is how I think about it:

• deg : maximal degree of GB elements?
• sel : ???
• pairs : number of unprocessed S-polynomials?
• mat/density : data for the current linear algebra solving stage?
• new data
  • new : number of new GB elements?
  • zero: ???
• time(rd) in sec (real|cpu) : timing for the current stage

[ederc]

94 adds an overview of the symbols stated.