Another attempt at fixing filtered PDF

1. Many compiler errors - undeclared variables, wrong indices etc.
2. The conditioning was being done on the unrotated velocity field. Changed to conditioning on the rotated field.
   This brings up another issue. The <u'w'> correlation will change because of the co-ordinate system rotation. This means that the updrafts and the downdrafts may not be so well correlated anymore or correlated better than before. Need to check this.
3. The real(nt*nxy) had to be replaced by dble(dble(nt)*dble(nnx)*dble(nny))