Add R^dxd and R^d expressions in pugs' language (with d=1,2 or 3)

These expressions are defined as follow

• for R^d: [a,b,...], where the d expressions (a,b,...) convert naturally to R

• for R^dxd: [[a,b,...],[c,d,...],...], where the dxd expressions (a,b,c,d,...) convert naturally to R

assigned to @delpino

enabled an automatic merge when the pipeline for 5c1cb487 succeeds

aborted the automatic merge because source branch was updated

added 1 commit

• fb973b01 - Change stringify utility to avoid a compiler bug in GitLab/CI

Compare with previous version

added 1 commit

• dfad0b1b - Rework vector<bool> conversion to string to satisfy libstdc++9

Compare with previous version

enabled an automatic merge when the pipeline for dfad0b1b succeeds

merged

mentioned in commit cf3cfef0