updates reading of external models (adds nummaterial_velocity_file); updates Par_files and examples (in particular Marmousi_mesh_of_the_model/ and MODEL = marmousi) #110

Job	Run time
Check file changes	16s
Test Intel on ubuntu-20.04	5m 13s
Test on Mac	7m 43s
Test on ubuntu-latest	1m 32s
Test on ubuntu-20.04	1m 18s
Test run example 2 - benchmark claerbout kernel elastic P-SV	2m 46s
Test run example 0 - make tests	2m 19s
Test run example 1 - benchmark claerbout kernel acoustic	2m 25s
Test run example 3 - benchmark claerbout kernel elastic SH	3m 7s
Test run example 4 - benchmark claerbout kernel acoustic-elastic	2m 48s
Test run example 5 - moving sources	1m 50s
Test run example 6 - anisotropy	1m 28s
Test run example 7 - Industrial_Format_SEP	4m 21s
Test run example 8 - axisymmetric_case_AXISYM_option	2m 11s
Test run example 10 - simple_topography_and_also_a_simple_fluid_layer debug double_precision	2m 18s
Test run example 11 - tomographic_ocean_model	1m 22s
Test run example 9 - simple_topography_and_also_a_simple_fluid_layer debug	2m 33s
	45m 30s

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