only ask for single line and single column of x and y for fgf property

RAM efficiency refactoring, pointed out by RKuehn

himawari/HimawariScene.py

@@ -205,9 +205,16 @@ class HimawariScene(object):
 
     @property
     def fgf(self):
+        y,_,_ = self.coords(columns=1, unscaled=True)
         meta = self.metadata
-        y,x,mb = self.coords(unscaled=True)
-        return fgf_yxmb(y[:,0].squeeze(),x[meta.begin_line,:].squeeze(),mb.my,mb.mx,mb.by,mb.bx)
+        _,x,mb = self.coords(line_offset=meta.begin_line, lines=1, unscaled=True)
+        return fgf_yxmb(y[:,0].squeeze(),x[0,:].squeeze(),mb.my,mb.mx,mb.by,mb.bx)
+
+    # @property
+    # def fgf(self):
+    #     meta = self.metadata
+    #     y,x,mb = self.coords(unscaled=True)
+    #     return fgf_yxmb(y[:,0].squeeze(),x[meta.begin_line,:].squeeze(),mb.my,mb.mx,mb.by,mb.bx)
 
     @property
     def line_times(self):