[raster] Modify ST_Mapalgebra so that the expression can refer to any band

[pracine]

This could be useful for:

-Writing expressions (like 'average') involving any band of the raster

-Refer to weighted information created by ST_AsRaster() (the length or the area of the geometry intersecting with the pixel

In the one raster version of MapAlgebra one could refer to the second band like this: 'rast[2]'.

In the two rasters version of MapAlgebra one could refer to the second band like this: 'rast1[2] + rast2[2]'.

'rast', 'rast1' and 'rast2' should by default refer to the value of the first band.

This change means that we MUST remove the function's band parameters since they are not useful anymore.