Add log Av grid spacing option

[karllark]

Log A(V) spacing enabled. This is picked by add adding a "log" as the 4th element of the vector defining the A(V) grid. E.g., [0.01, 10.0, 0.1, "log"] to get a logAv spacing of 0.1 between 0.01 and 10.0.

This required adding in grid weights for all the dust parameters. Previously, these were always the same given a linear grid was used implicitly.