Fix poorly displayed matrices by replacing text-mode display_matrix
function with math-mode display_matrix_mm function.

These problems used the text-mode display_matrix function in PGmatrixmacros.pl to
display matrices.  \{display_matrix($m)\} inserts very little space between the
columns of $m so many matrices are hard to read.  Replacing it with the
math-mode  \( \{display_matrix_mm($m) \}\) cures the problem.
Deleted a file that's not in the official repository.
Merge pull request #61 from Dick-Lane/master

Both parts look like improvements.
New optimization problem.
Merge pull request #60 from gajennings/master

Fix column spacing in matrix problems
Add clarifying instructions to problem text

Inform students that both answer blanks must have answers for the answer checker to work.
Make answer checker more robust and fix wording
Fix ImplicitEquation answer checker in part b.

Change the domain of function evaluation and provide some exact solutions to check in part b.
Range of randomization reduced. Answer box increased.
Fix bug 3027
Correct answer
Change broken units checking to pop-ups.
Merge branch 'master' of github.com:openwebwork/webwork-open-problem-library
Small fix.
Replace  black  with  'black'  to avoid a warning message:
	Unquoted string "black" may clash with future reserved word
	at line 61 of (eval 6877)

curious: warning if redirected from Q23.pg but not if loaded directly