Respect env variables upon activation/deactivation

[pmav99]

activate.sh should respect the existing values of PYTHONPATH and LD_LIBRARY_PATH

telemac-mascaret-feedstock/recipe/scripts/activate.sh

Lines 17 to 20 in df2599a

	export PATH=$HOMETEL/scripts/python3:$PATH
	export PYTHONPATH=$HOMETEL/scripts/python3
	export PYTHONPATH=$HOMETEL/builds/$USETELCFG/wrap_api/lib:$PYTHONPATH
	export LD_LIBRARY_PATH=$HOMETEL/builds/$USETELCFG/wrap_api/lib:$HOMETEL/builds/$USETELCFG/lib:$CONDA_PREFIX/lib

while deactivate.sh should restore the "old" values of PATH, PYTHONPATH and LD_LIBRARY_PATH and PYTHONUNBUFFERED

telemac-mascaret-feedstock/recipe/scripts/deactivate.sh

Lines 4 to 6 in df2599a

	unset PYTHONUNBUFFERED
	unset PYTHONPATH
	unset LD_LIBRARY_PATH

This might be useful for implementing this: https://stackoverflow.com/a/47159781
Alternatively the "before-activation" values need to be stored in a different env variable and get restored upon deactivation.

[pmav99]

For the record, unless PYTHONUNBUFFERED is somehow required/expected by telemac, I would avoid setting it in the first place. Let the user choose to set it (e.g. via python -u or something). Silently changing default behaviors can be hard to debug.