`onestep` model: alleles without neighbours

[thoree]

Maybe rather an error indicating that neigbors are needed:

pedmut::mutationModel("onestep", alleles = c(1:2,4), rate = 0.002)
#>     1     2     4 
#> 1.000 1.000 0.998
#> Error: Rows which do not sum to 1 (after rounding to 3 decimal places): 3

^{Created on 2019-10-28 by the reprex package (v0.3.0)}

[magnusdv]

I've pushed a temporary fix for this, which leaves the 4 allele immutable (since it doesn't have any immediate neighbors to mutate to).

Keeping the issue open, in case we think of a better way to implement this.