Add getindex method to LU{Tridiagonal} for extracting factors (and full)

[andreasnoack]

This one has been missing for a while. Fixes #11300, but returns a Matrix because the factors have to be stored as Matrixes. I don't think it is worth the trouble to make full return a Tridiagonal. I've added a specialized full method that returns the original Tridiagonal matrix.

[tkelman]

full returning a Tridiagonal is a little strange

[andreasnoack]

For factorizations, I think of full as the way of getting the original matrix back. We could consider other names for that.

[tkelman]

convert ?

[andreasnoack]

No because then you'd have to specify a type.

Den tirsdag den 14. juli 2015 skrev Tony Kelman notifications@github.com:

convert ?

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#12137 (comment).

[tkelman]

That seems like a good thing - the issue here is you've overloaded full with different ambiguous meanings. I think of full as always returning a dense array. This "inverse of factorize" meaning is really a totally separate thing.

[tkelman]

opened #12153