Bug fix: return string from get_xc_functional
Some ASE functionality (at least vdWTkatchenko09prl in vdwcorrection.py) assume that calculator.get_xc_functional
return the XC name as string
Some ASE functionality (at least vdWTkatchenko09prl in vdwcorrection.py) assume that calculator.get_xc_functional
return the XC name as string