pass the order variable to MOLFiniteDifference

[j-bowhay]

Checklist

• Appropriate tests were added
• Any code changes were done in a way that does not break public API
• All documentation related to code changes were updated
• The new code follows the
  contributor guidelines, in particular the SciML Style Guide and
  COLPRAC.
• Any new documentation only uses public API

Additional context

I noticed a few of the tutorials define an order variable but then never uses it which is mildly confusing to read. In this pr is pass this variable to the approx_order argument of MOLFiniteDifference.

[ChrisRackauckas]

In all of the cases here the order is just the default approximation order. Is it easier to read just removing it?

[j-bowhay]

In all of the cases here the order is just the default approximation order. Is it easier to read just removing it?

Equally happy to do that but I thought maybe it would be worth keeping to draw attention to the fact you can change it?

[ChrisRackauckas]

That's why it's a question. Unsure. @xtalax your take?