utility to rename existing fields

Useful, for example, to change a UMean to U field for further calculation (ref: EP523)

• if required in the future, could have a generalized rename utility (with regular expressions and placeholders etc), but for the current requirements a simple small set of pre-defined methods should be sufficient.

• The method mean

    s/^(.+)Mean/$1/;
    # Renames existing target as  $1.orig

• The method orig

    s/^(.+)\.orig/$1/;

These predefined actions are obviously less flexible than regex support, but presumably also user friendlier.

@Prashant