Add ability to set NullCellValue for .ers files using ERMapper driver

[dougc]

The .ers NullCellValue header field can be read but not written by GDAL. Our application needed a way to set this. Since the GDAL NoDataValue methods are implemented per band and not by the driver, it seems like a driver option might be a good way to enable this capability. I implemented the code below in a private build and would like to suggest it for inclusion in GDAL. Based on existing .ers files written elsewhere that have NullCellValue set, I think it's best placed in ersdata.cpp after the CellType field written at line 1148 of rev 17119. Thanks for considering this.

/* -------------------------------------------------------------------- */
/*      Write the null cell value if supplied.                          */
/* -------------------------------------------------------------------- */
	const char * pszNullCellValue = CSLFetchNameValue( papszOptions, "NULLCELLVALUE" );
	if( pszNullCellValue )
	{
		VSIFPrintfL( fpERS, "\tNullCellValue\t= %s\n", pszNullCellValue );
	}

[dougc]

This was resolved in 1.8.1 via #4207.