improve checkparset error message

Improves the error/warning message printed when DP3 checks and finds unused parset keywords. I noticed that if there are are >1 keywords found, they are not separated at all. That gets unclear fast thus.

This MR changes the behavior from e.g.:

E                   *** WARNING: the following parset keywords were not used ***
E                                maybe they are misspelled
E                       solve_complexgain3.beam_intervalsolve_scalarphase3.beam_interval

To:

E                   *** WARNING: the following parset keywords were not used ***
E                                maybe they are misspelled
E                       - solve_complexgain3.beam_interval
E                       - solve_scalarphase3.beam_interval