Make sure "post" array is reversed relative to "pre" array

[bpj]

When formatting/concatenating the latex "post" markup we currently rely on the items in the post array to be just strings of closing curly brackets so that it doesn't matter in which order they are concatenated. To be safe against future additions to the post templates and against people copying the "algorithm" we should add a reverse_array helper function and call it on the post array before concatenating it.

local function reverse_array (arr)
  if 'table' == type(arr) then
    local rev = {}
    for i=#arr, 1, -1 do
      rev[#rev+1] = arr[i]
    end
    return rev
  end
  error"Expected argument to be table"
end

  post = table.concat( reverse_array(post) )