BundleSolver setup too restrictive about neurodiffeq

[shuheng-liu]

Plus, here's my interpretation of line 1331. It might be helpful to anyone else trying to understand what the non_var_filter is doing.

• len(conditions) is the number of functions to solve for
• + 1 is hard-coded number of free variables/coordinates/inputs to functions. Since this is an ODE solver, there's only 1 input.
• + n is the index of a var (a parameter that appears only in the condition). It's deleted since the var doesn't appear in the equation by assumption.
• - i accounts for the fact that at step i (starting from 0), a total of i vars have been previously deleted. This assumes previously deleted vars appear before the current var (i.e., non_var list is sorted)! Which is not guaranteed by line 1326 and can lead to unnoticeable bugs!