Update the priority of steps for finding the MOPAC executable
Update the priority of steps for finding the MOPAC executable

Previously, we defaulted to the /opt/mopac directory that had mopac.
Now, we want to instead find it based off the conda package instead
since most people will be running using anaconda. This also
helps let users update their MOPAC package more easily
if expired.

chatelak/RMG-Py,pierrelb/RMG-Py,nyee/RMG-Py,nickvandewiele/RMG-Py,pierrelb/RMG-Py,nyee/RMG-Py,chatelak/RMG-Py,nickvandewiele/RMG-Py