Point to cov if not positive definite (#746)

Author: rikhuijzer

src/reliability.jl

@@ -51,7 +51,11 @@ item 4: 0.7826
 ```
 """
 function cronbachalpha(covmatrix::AbstractMatrix{<:Real})
-    isposdef(covmatrix) || throw(ArgumentError("Covariance matrix must be positive definite."))
+    if !isposdef(covmatrix)
+        throw(ArgumentError("Covariance matrix must be positive definite. " *
+                            "Maybe you passed the data matrix instead of its covariance matrix? " *
+                            "If so, call `cronbachalpha(cov(...))` instead."))
+    end
     k = size(covmatrix, 2)
     k > 1  || throw(ArgumentError("Covariance matrix must have more than one column."))
     v = vec(sum(covmatrix, dims=1))