Add basic test for correct fixed effect matrix

test/runtests.jl

@@ -1,6 +1,10 @@
 using BeefBLUP
+using DataFrames
 using Test
 
 @testset "BeefBLUP.jl" begin
     # Write your tests here.
+    correctX = [1 1 0 0; 1 1 0 1; 1 0 1 0; 1 0 1 1; 1 0 1 0; 1 0 1 1; 1 0 0 0]
+    fixedfx = DataFrame(year = [1990, 1990, 1991, 1991, 1991, 1991, 1992], sex = ["male", "female", "male", "female", "male", "female", "male"])
+    @test BeefBLUP.fixedeffectmatrix(fixedfx)[1] == correctX
 end