Tidy up cov scal

I've tidied up the analysis program in cov_scal.f90 a bit by removing redundant variables and a do- and an If-structure that do nothing. Also, the previous program skipped the first n_skip-1 instead of the first n_skip bins. I've also added a file ".gitignore", which tells git to ignore all files ending in .o, .mod, .out and .a, to prevent from accidentally adding such files to the git.