adds full gravity seismograms (gravitational potential, gravity perturbation, etc.) #177

Job	Run time
Check file changes	19s
Test Intel on ubuntu-20.04	10m 31s
Test on ubuntu-latest	2m 36s
Test on Mac	7m 58s
Test on ubuntu-20.04	2m 0s
Test run example 2 - regional_Greece_small	3m 46s
Test run example 1 - regional_Greece_small debug	2m 2s
Test run example 3 - regional_sgloberani	3m 42s
Test run example 0 - make tests	10m 16s
Test run example 10 - regional_Greece_small_LDDRK kernel ADIOS2	11m 6s
Test run example 4 - point_force	4m 58s
Test run example 5 - moon_global	5m 18s
Test run example 7 - global_small	5m 18s
Test run example 6 - mars_regional	3m 15s
Test run example 8 - global_small w/ FULL_GRAVITY netCDF	11m 3s
Test run example 9 - global_small w/ FULL_GRAVITY PETSc	5m 3s
	1h 29m 11s

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