[raster] Modify ST_MapAlgebra so that expressions 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.

Out of bound band references should be clamped to the existing band range.

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