[BUG] Rename `hess` field into `hess_inv` for result of BFGS minimization

Also document the fact that the Hessian inverse returned by BFGS is
approximated.

Closes #2647.
BUG: arpack-ng: fix bug occurring on OSX or LAPACK 3.2.1

See discussion at http://forge.scilab.org/index.php/p/arpack-ng/issues/1259/

The Ritz vector purification step assumes workl(iq) still contains the
original Q matrix. This is however overwritten by the call to xGEQR2
earlier.

This patch fixes the issue by making a copy of the last row of the eigenvector
matrix, after it is recomputed after QR by xORM2R. The work space
WORKL(IW+NCV:IW+2*NCV) used for the TAU values of the QR decomposition is not
used later in the routine, and can be used to store a copy of the last row.

Fixes gh-2547 (provided you use -ff2c flag for compiling)

Thanks to Michael Wimmer for tracing the issue.
DOC: remove backticks on None, False, True
DOC: stats: Several corrections to the betaprime docstring:
* fixed a typo;
* fixed a mistake in the formula for the PDF;
* expressed the PDF in terms of beta(a,b) instead of the gamma function.
Add try/except clause around setting old_argv in case sys.argv is not defined.

sys.argv may not be defined if Python interpreter is embedded in another program.
MAINT: update author name mapping in tools/authors.py
Merge pull request #2688 from WarrenWeckesser/stats-betaprime-typos

DOC: stats: Several corrections to the betaprime docstring:
Merge pull request #2693 from ajdunlap/master

Add try/except clause around setting old_argv in case sys.argv is not defined.
Merge pull request #2684 from pv/arpack-fix

BUG: arpack-ng: fix bug occurring on OSX or LAPACK 3.2.1
Merge pull request #2686 from sciunto/doc_backticks

DOC: remove backticks on None, False, True
DOC: optimize: warn about approximate hessians
Merge pull request #2659 from dlax/issue-2647

BUG: optimize: Rename `hess` field into `hess_inv` for result of BFGS minimization

Also warn in the documentation that these may be just approximations.