update solutions

The QR was incorrectly calculated.
reorganized QR explanation

Added a problem to use QR to calculate the determinate.
revisited least squares with QR.

Use triangular solver instead of inverting R.  Updated solutions.
rename QR tex file
talk about pivoting
add QR least squares problem

Use QR decomposition to computer solutions to least squares problems.
remove section on pivoting

Also added comments to solutions file.
Merge pull request #324 from ayr0/qr

Added problems related to QR and least squares using QR.