added -j8 option for faster plumed make

if buildflag:

   print("Building plumed ...")

   cmd = 'cd %s/plumed-%s; ./configure --prefix=%s --enable-static-patch ; make ; make install' % (homepath,version,homedir)

   cmd = 'cd %s/plumed-%s; ./configure --prefix=%s --enable-static-patch ; make -j8 ; make install' % (homepath,version,homedir)

   txt = subprocess.check_output(cmd,stderr=subprocess.STDOUT,shell=True)

   print(txt.decode('UTF-8'))

#