Add fill_mapping_field_from_mapping_file

This function will padd a metadata mapping file with given values in the
format of Category:ValueToPad;OtherCategory:OtherValueToPad.
Add --missing_custom_axes_values option

This option overrides the error raised when mapping file values are
passed with non-numeric values, the user is now capable of selecting
what this values are. Instead of removing the samples.