Update rotational_utils.jl

Change back to Lukas' implementation so that we don't use fill, which would create an array on CPU.

src/rotational_utils.jl

@@ -1,11 +1,10 @@
 # from AF2 supplementary: Algorithm 23 Backbone update
 """
-Takes a 3xN matrix of imaginary quaternion components and returns a 4xN matrix of unit quaternions.
+Takes a 3xN matrix of imaginary quaternion components, `bcd`, sets the real part to `a`, and normalizes to unit quaternions.
 """
-function bcds2quats(bcd::AbstractMatrix{T}) where T <: Real
-    quats = vcat(ones(T, 1, size(bcd, 2)), bcd)
-    norms = sqrt.(one(T) .+ sum(abs2, bcd, dims=1))
-    return quats ./ norms
+function bcd_to_quat(bcd::AbstractMatrix{T}, a::T=T(1)) where T <: Real
+    norms = sqrt.(a .+ sum(abs2, bcd, dims=1))
+    return vcat(a ./ norms, bcd ./ norms)
 end
 
 """