GMGPolar deconstructor uses levels_orig: Can cause segfaults in unit tests

[mknaranja]

The GMGPolar deconstructor uses levels_orig https://github.com/mknaranja/GMGPolar/blob/main/src/gmgpolar.cpp#L51 which only is set in define_coarse_nodes.cpp https://github.com/mknaranja/GMGPolar/blob/main/src/define_coarse_nodes.cpp#L52. This causes a segfault in the tests which was 'workarounded' with #44. A better strategy of default initialization or removal of levels_orig should be considered.