readd sum checks. I am not sure where we lost those.

reintroduce them from 2020.

src/manifolds/MultinomialSymmetric.jl

@@ -56,7 +56,22 @@ i.e. is a symmetric matrix with positive entries whose rows sum to one.
 """
 function check_point(M::MultinomialSymmetric, p; kwargs...)
     n = get_parameter(M.size)[1]
-    return check_point(SymmetricMatrices(n, ℝ), p)
+    s = check_point(SymmetricMatrices(n, ℝ), p)
+    isnothing(s) && return s
+    r = sum(p, dims=2)
+    if !isapprox(r, ones(n, 1); kwargs...)
+        return DomainError(
+            r,
+            "The point $(p) does not lie on $M, since its rows do not sum up to one.",
+        )
+    end
+    if !(minimum(p) > 0) || !(maximum(p) < 1)
+        return DomainError(
+            minimum(p),
+            "The point $(p) does not lie on $M, since at least one of its entries is nonpositive.",
+        )
+    end
+    return nothing
 end
 @doc raw"""
     check_vector(M::MultinomialSymmetric p, X; kwargs...)
@@ -67,7 +82,16 @@ along any row.
 """
 function check_vector(M::MultinomialSymmetric, p, X; kwargs...)
     n = get_parameter(M.size)[1]
-    return check_vector(SymmetricMatrices(n, ℝ), p, X; kwargs...)
+    s = check_vector(SymmetricMatrices(n, ℝ), p, X; kwargs...)
+    isnothing(s) && return s
+    r = sum(X, dims=2) # due to symmetry, we only have to check columns
+    if !isapprox(r, zeros(n); kwargs...)
+        return DomainError(
+            r,
+            "The matrix $(X) is not a tangent vector to $(p) on $(M), since its columns/rows do not sum up to zero.",
+        )
+    end
+    return nothing
 end
 
 embed!(::MultinomialSymmetric, q, p) = copyto!(q, p)
@@ -115,8 +139,7 @@ The two vector $α ∈ ℝ^{n×n}$ is given by solving
 ````math
     (I_n+p)α =  Y\mathbf{1},
 ````
-where $I_n$ is teh $n×n$ unit matrix and $\mathbf{1}_n$ is the vector of length $n$ containing ones.
-
+where ``I_n`` is teh ``n×n`` unit matrix and ``\mathbf{1}_n`` is the vector of length ``n`` containing ones.
 """
 project(::MultinomialSymmetric, ::Any, ::Any)