Switch to P2 default encounter rate variance for point transects

[dill]

To match Distance for Windows, per changes from Fewster et al (2009).

Reference #135

[LHMarshall]

The documentation details that the P2 is now the default since 1.0.9 and that P3 was the default prior to this. No warnings will be given to the user. I do not think there is any way to know if the user specified P2 or it was used because it is now the default. We would either have to display a warning to all who were analysing point transect data or none.

[LHMarshall]

Loading Distance loads mrds and displays the message so separate message not required for Distance