parser missing input fix, parser accepts assignable calls with multiple variable inputs

• Fixed an error where the top most sub group was not checked for missing inputs
• The parser now accepts calls with multiple variable inputs IF there is only one and only one way the input could be interpreted.
  Aborts the moment there is a variable input operator without a subgroup and more than 1 child.
• Improved the error messages for missing inputs and variable inputs without subgroups.
  They now state that the node is not assignable and the reason why.
• Added tests for the fixed missing check for missing input and updated the test to allow the multiple variable input changes