Poloidal magnentic field warning should be output at run time, not at compile time

The warning

/home/jenkins/workspace/EinsteinToolkit/arrangements/EinsteinInitialData/GRHydro_InitData/src/GRHydro_PoloidalMagFieldM.F90:110:2: warning: #warning "This algorithm does only work on Cartesian grids!!" [-Wcpp]

should probably be output at run time, not only at compile time.

(Of course, it would be even better if the code looked at the grid structure to determine whether the grid is Cartesian.)

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