Cleanup ipopt solvers

See merge request !48

removing redundant IPOPTSolver implementation

APIADD: intermediate callback can be specified with
void set_callback_function
(std::function<bool(const IPOPTCallbackParameters &)>)

APICHG: renamed NProblemIPOPT -> IPOPTProblemInstance
since it does not implement the NProblemInterface and should not be
used by clients of CoMISo directly

this allows string literal parameters to decay to std::string implicitly

APICHG: rename getters to use get_ consistently

unify method signature formatting
cleanup whitespace
obey 80 character limit

Cmake overhaul

See merge request !47

preparing merge into master

add CoMISo subdir to INSTALL_INTERFACE includes
unconditionally add COMISO_INCLUDE_DIRECTORIES
delete CoMISoConfig.cmake from repo
instead export targets directly to CoMISoConfig.cmake

COMISOSolver: warn about non-equality constraints

See merge request !45

QuadraticProblem: add getters.

See merge request !46

cmake: do not try to install non-existnt generated Config.cmake, install cmake files in INSTALL_CONFIGDIR

- always set project name
- be more careful setting include directories
- install targets and version file, export package
- CMakeConfig file: only include targets.cmake if target does not exist yet.

- find any version of GUROBI based on the GUROBI_HOME env var (suggested
by Gurobi docs).
- compile C++ library ourselves to avoid toolchain/ABI mismatches

Symmetric dirichlet problem

See merge request !43