update GWAS: fixed data type for 2nd and 3rd column when read mapfile.

[zhaotianjing]

Fixed data type for 2nd and 3rd column when read mapfile.

Reason: user reported that old code mapfile = CSV.read(map_file, DataFrame, header = header, types=Dict(1 => String)) converts the 3rd column to a crazy type of ::SentinelArrays.ChainedVector{Float64, Vector{Float64}}). This will cause error for this line in GWAS(): map(Int64,mapfile[:,3]).

The new code solves the issue. New code: mapfile = CSV.read(map_file, DataFrame, header = header, types=Dict(1 => String, 2 => String, 3 => Int64))

Other modification:
old code:

    chr     = map(string,mapfile[:,2])
    pos     = map(Int64,mapfile[:,3])

new code:

    chr     = mapfile[:,2]
    pos     = mapfile[:,3]