pair_pace.cpp: check that units are "metal"

update ace-evaluator (download link + md5sum in cmake and make build systems):  accept multilines comment at the beginning of potential.ace file
add first comment line for potentials/Cu-PBE-core-rep.ace

lib/pace/Install.py

@@ -12,20 +12,22 @@ from argparse import ArgumentParser
 sys.path.append('..')
 from install_helpers import fullpath, geturl, checkmd5sum
 
-parser = ArgumentParser(prog='Install.py',
-                        description="LAMMPS library build wrapper script")
-
 # settings
 
 thisdir = fullpath('.')
-version = "v.2021.2.3.upd2"
+version = 'v.2021.4.9'
 
 # known checksums for different PACE versions. used to validate the download.
 checksums = { \
-        'v.2021.2.3.upd2' : '8fd1162724d349b930e474927197f20d', \
+        'v.2021.2.3.upd2' : '8fd1162724d349b930e474927197f20d', 
+        'v.2021.4.9'      : '4db54962fbd6adcf8c18d46e1798ceb5',          
         }
 
 
+parser = ArgumentParser(prog='Install.py',
+                        description="LAMMPS library build wrapper script")
+
+
 # help message
 
 HELP = """