invertAlpha switch in setAlphaField

For simulating rising bubbles etc it is convenient to be able to invert alpha so e.g. the sphere/disc formed with setAlphaField has alpha = 0 inside and is surrounded by alpha = 1.

In the attached file setAlphaField is the version from v1806 where I have added this functionality. A discInConstantFlow case is added to verify that it works as intended.

Best regards, Johan