Created --roverfile parameter and fixed output bug

BUG: Rover output file not cleared at beginning of each run of script.

SOLUTION: Created parameter for roverfile output, moving file creation
out of the print_best_primers function (which was called multiple times
during a single run, resetting the file OR appending incorrectly
depending on the file open argument) into the parameters.

Parameters are called only once, facilitating correct behaviour.