Remove invalid $PnE output

[zbjornson]

From the FCS3.1 spec:

$PnE If the floating point data type is used (either $DATATYPE/F/ or $DATATYPE/D/), then all parameters shall be stored as linear with $PnE/0,0/.

and

$DATATYPE Note that $PnE/0,0/ shall be used for all parameters if $DATATYPE/F/ is used.

The part of the spec that talks about invalid $PnE values is providing recommendations to file readers. Writers should just not make invalid files.

I made this a subclassed warning so it can easily be silenced by users, but could just make it silently ignored also.