implement :det_exact and :inverse_exact for list/yale matrices

[spacewander]

This pr is about issue #189, which succeeds #334

[translunar]

@spacewander Can you include some specs with Yale 3x3 matrices with more zeros? I think your tests are providing full coverage, but I'm not one hundred percent certain.

[spacewander]

No problem.

[translunar]

@spacewander Any progress on this?

[spacewander]

Oh yes, I have added a new case to this pr:

c = NMatrix.new(3, [1,0,3,0,0,1,0,6,0], stype: :yale, dtype: :int64)
inversed = c.method(:__inverse_exact__).call(c.clone, 3, 3)
d = NMatrix.new(3, [1,-3,0,0,0,0,0,1,0], stype: :yale, dtype: :int64)
expect(inversed).to eq(d)

And the code can work properly.

Is it enough?

[translunar]

square dense

[translunar]

Overcome by #580.