netdist function updates:
- method parameter changed in d
- added more aliases for d values
- added check on the allowed parameters
- updated netdist doc accordingly (possibly move all the aliases in the details section)

NOTE: still present a bug on passing ga to netdist (both NULL and an integer) triggering
Error in UseMethod("g2adj") :
  no applicable method for 'g2adj' applied to an object of class "list"
(strange behavior since if debugging the g2adj/g2adj.matrix functions doesn't appear)

NOTE2: netSI must be updated accordingly
Merge branch 'dev' of https://github.com/filosi/nettools into dev