Feature/switch dep check and set cxx flags

Four new functions added to dune.generator to disable dependency checking during make (i.e. unconditionally make the target) and to change the cxx flags - mostly useful together, i.e., to recompile one module in debug mode. Can be used directly in a scipt by calling for example

dune.generator.setFlags("-g") dune.generator.setNoDependencyCheck()

and then

dune.generator.unsetFlags() # to get the flag used during configu dune.generator.setDependencyCheck()

before/after a module is compile, e.g., calling dune.grid.yaspGrid(...)

These can be simplified by using

dune.generator.setFlags("-g",noCheck=True)

and

dune.generatpor.reset()