Added Python version of mkpotcar

Currently named mkpotcar.py
This breaks compatibility with how the environment variable
POTCARDIR is used. For this version of the code, the
pseudopotential set is left out (e.g. 'PBE_54' ), and then
added automatically in the script. This can be switched
using the '--potcar-set' flag.

mkpotcar.py

@@ -0,0 +1,28 @@
+#! /usr/bin/env python3
+
+import os
+import argparse
+import sys
+
+parser = argparse.ArgumentParser( description='Construct a VASP POTCAR file' )
+parser.add_argument( 'potcars', metavar='P', type=str, nargs='+', help='the string identifying each POTCAR directory.' )
+parser.add_argument( '--potcar-set', default='PBE_54', help='choose which POTCAR set to use' )
+args = parser.parse_args()
+
+try:
+    potcardir = os.path.join( os.environ['POTCARDIR'], args.potcar_set )
+except KeyError:
+    raise( '$POTCARDIR is not set' )
+
+if not os.path.isdir( potcardir ):
+    raise ValueError( 'Cannot find directory f{potcardir}' )
+
+for potcar in args.potcars:
+    if not os.path.isdir( os.path.join( potcardir, potcar ) ):
+        print( '{} not found in {}'.format( potcar, potcardir ) )
+        print( 'Available pseudopotentials ({})'.format( args.potcar_set) )
+        os.system( 'get_potcar_list.py' ) # This should be a module call instead.
+        sys.exit()
+
+for potcar in args.potcars:
+    os.system( 'cat {}/POTCAR'.format( os.path.join( potcardir, potcar ) ) )