Script `07`: issues running `segsmooth`

[alonsoJASL]

Raised by @MarinaStrocchi:

• In some cases, the script does not find segsmooth in the PYTHONPATH even when the environment is set correctly. The command works on the terminal directly, but not through the python script.
• It seems that the SEGTOOLS_DIR in the 07-segmt-smoothing.sh script does not accept an empty string.