Add an ESRI ASCII Grid reader and writer

Add a reader and writer for ESRI ASCII Grid format. This is a very simple format easy to implement, and used commonly enough for being worth to be supported. See the Wikipedia page for a format description.

The reader/writer to implement in SIS are ports from the Geotk AsciiGridReader and AsciiGridWriter classes. Compared to the ESRI ASCII Grid format, the SIS implementation will have the following extensions. The 2 first extensions in this list exist also in GDAL:

• Coordinate reference system specified by auxiliary *.prj file. If the format is WKT 1, the GDAL variant is used (that variant differs from the OGC 01-009 standard in their interpretation of units of measurement).
• DX and DY parameters in the header are used instead of CELLSIZE if the pixels are non-square.
• Lines in the header starting with '#' are ignored as comment lines.