Addresses #187 by correctly outputting a matrix from an asc file

From now on (refer to the issue for context):
- write grid_data.contents will return a geometry (the enclosing envelope)
- write grid_data as matrix will return the matrix<float> of the values
- write grid_data as field will return the same values

gama.core/src/gama/core/util/file/GamaGridFile.java

@@ -64,6 +64,7 @@
 import gama.core.util.matrix.GamaField;
 import gama.core.util.matrix.GamaFloatMatrix;
 import gama.core.util.matrix.IField;
+import gama.core.util.matrix.IMatrix;
 import gama.gaml.statements.Facets;
 import gama.gaml.types.GamaGeometryType;
 import gama.gaml.types.IType;
@@ -353,7 +354,7 @@ private void customAscReader(final IScope scope) {
 						ascInfo[2] = xCorner;
 					} else if (yCorner == null && yCenter == null && line.contains("yllcorner")) {
 						yCorner = doubleVal(line);
-						//TODO: very suspicious, probably xllcenter and yllcenter 
+						// TODO: very suspicious, probably xllcenter and yllcenter
 					} else if (xCorner == null && xCenter == null && line.contains("xllcorner")) { // AD To verify: the
 																									// conditions are
 																									// the same as two
@@ -849,4 +850,11 @@ public double[] getBand(final IScope scope, final int index) {
 		return Arrays.copyOf(records.bands.get(index), length(scope));
 	}
 
+	@Override
+	protected IMatrix _matrixValue(final IScope scope, final IType contentsType, final GamaPoint preferredSize,
+			final boolean copy) throws GamaRuntimeException {
+		getContents(scope);
+		return new GamaField(scope, this);
+	}
+
 }