Extend LauArgusPdf to the generalised form where p is not fixed to 1/2

[Thomas Latham]

The form for the ARGUS function in Laura++ is currently that where the power is fixed to 1/2. We can extend to allow this to take other values (and indeed to be floated). The complication is that the analytical integral only works for this special case, so we'd have to check whether p=1/2 and is fixed, otherwise revert back to numerical integration.