Switch to P2 default encounter rate variance for point transects

[dill]

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

Reference DistanceDevelopment/Distance#135

[LHMarshall]

@lenthomas are we just going to warn about this change in the announcements? We don't want to warn when the analyses are run do we?

[lenthomas]

Suggest we put a notice about this on startup for mrds and Distance for this release and then remove again at a later time.