The quadratic ipopt solver can use a variety of linear solvers. The
ipopt library itself does not contain a single one; they are dlopen'ed
at run-time.

The wrapper class in this module takes MA27 from libhsl to be the
default solver. This is also the default solver in ipopt, so that
setting linear_solver to MA27 has the same effect as not setting it at
all. Setting it to the empty string appears to have the same effect but
triggers a warning.

libhsl (which is installed under the name libcoinhsl by default even
though dlopen requires it to be called libhsl) is not available under a
permissive license but free for researchers.

Even though the quadratic ipopt solver class itself could be used with
other linear solvers (from mumps or pardiso) in theory (I haven't
checked), the test will only run with libhsl for now. Unless both ipopt
and libhsl are found, the test will now be skipped.