Add another testcase.

This is for the same problem as reported by Krishna Garikipati
on the mailing list this week. He verified that this used to fail
and works now, so I would like to add it to the testsuite to make
sure we don't regress in this functionality.
Merge pull request #299 from bangerth/add-test-2

Add another testcase.
Reorganize code a bit.

Change the code so that checking whether a function parser object
has already been created now happens at the place where we use
the object, rather than as an early exit in the init_muparser()
function. I find that this makes the code easier to read.

This also allows us to remove the creation of a parser object on the
thread where the object is actually created. In many (most) cases,
we will simply create the parser later on anyway, but not necessarily.
Clean up documentation a bit.
Make an error message easier to read.

Enclose strings in <...> so that one can more easily see if there happen
to be spaces in untoward places.
Merge pull request #301 from bangerth/muparser-cleanups

Muparser cleanups