Allow paths/filenames with spaces when calling samtools

In createTHetAExomeInput.py, quote the filename arguments inside each call to os.system to prevent the shell from parsing the space-containing argument as multiple tokens (i.e. garbage).

Alternative solutions not attempted here:
1. use subprocess & avoid using the shell altogether; or
2. use pysam & avoid creating the pileup files on disk.

Also fixed a typo.