add bootgen2*() functions

[zkamvar]

Brought up in #111, it's apparent that aboot() will fail if the distance function has stopifnot(is.genind(x)) at the beginning of the function. To alleviate this, I'll provide a converter so that the user can specify

aboot(mygenind, distance = function(i) mydist(bootgen2genind(i)))

and it will work.

Additionally, documentation needs to be updated to specify that the incoming genind object must have the same dimensions as the outgoing tree object (in strata or individuals).