document the fully_determined kwarg to structural_simplify

[MasonProtter]

This provides a bit more information about the options available to structural_simplify.

Before:

help?> structural_simplify
search: structural_simplify structural_simplify!

  structural_simplify(sys; ...)
  structural_simplify(sys, io; simplify, split, kwargs...)
  

  Structurally simplify algebraic equations in a system and compute the topological
  sort of the observed equations. When simplify=true, the simplify function will be
  applied during the tearing process. It also takes kwargs allow_symbolic=false,
  allow_parameter=true, and conservative=false which limits the coefficient types
  during tearing. In particular, conservative=true limits tearing to only solve for
  trivial linear systems where the coefficient has the absolute value of 1.

  The optional argument io may take a tuple (inputs, outputs). This will convert all
  inputs to parameters and allow them to be unconnected, i.e., simplification will
  allow models where n_unknowns = n_equations - n_inputs.

after:

help?> structural_simplify
search: structural_simplify structural_simplify!

  structural_simplify(sys; ...)
  structural_simplify(
      sys,
      io;
      simplify,
      split,
      allow_symbolic,
      allow_parameter,
      conservative,
      fully_determined,
      kwargs...
  )
  

  Structurally simplify algebraic equations in a system and compute the topological
  sort of the observed equations in sys.

  Optional Arguments:
  –––––––––––––––––––

    •  optional argument io may take a tuple (inputs, outputs). This will
       convert all inputs to parameters and allow them to be unconnected, i.e.,
       simplification will allow models where n_unknowns = n_equations -
       n_inputs.

  Optional Keyword Arguments:
  –––––––––––––––––––––––––––

    •  When simplify=true, the simplify function will be applied during the
       tearing process.

    •  allow_symbolic=false, allow_parameter=true, and conservative=false limit
       the coefficient types during tearing. In particular, conservative=true
       limits tearing to only solve for trivial linear systems where the
       coefficient has the absolute value of 1.

    •  fully_determined=true controls whether or not an error will be thrown if
       the number of equations don't match the number of inputs, outputs, and
       equations.

[ChrisRackauckas]

Actually, let me take a shot at sorting this a bit.

[ChrisRackauckas]

did you just delete my changes?

[MasonProtter]

Oh crap, did I? I didn't know you were pushing directly to my branch I'm sorry. Must have happened when I was trying to fix the merge conflict

[MasonProtter]

you can re-force push them if you still have them locally

[ChrisRackauckas]

this is good.