Updated example scripts and bugfix for TPXO environment variables

[cuill]

Bugfix:

• gen_hrrr3.py - fixed the issue "bbox is not defined"
• fixed issue #118

Added more scenarios in download_hycom.py

Removed a repeated variable in hycom2shcism.py

Linked issues:

Closes https://github.com/schism-dev/pyschism/issues/118