add costs of dispersal?

[mdahirel]

Some results from test runs seem different from existing lit where they should not. Lack of dispersal costs here may be the cause.

This issue is a reminder to think about whether it'd be justified/useful to add dispersal costs:

• either as mortality costs (as in Travis et al 09)
• or as a fecundity penalty in new patch

[mdahirel]

mortality costs now possible as of e9af15064b04182e8c90a7af8646c3c7c070b3b5