adds possibility to read external source time function files in binary format #201

Job	Run time
Check file changes	28s
Test Intel on ubuntu-20.04	13m 28s
Test on Mac	18m 14s
Test on ubuntu-latest	2m 6s
Test on ubuntu-20.04	2m 8s
Test run example 1 - meshfem3D simple model	3m 54s
Test run example 3 - layered halfspace	3m 23s
Test run example 2 - fault tpv5	5m 32s
Test run example 11 - poroelastic	3m 10s
Test run example 0 - make tests	7m 3s
Test run example 12 - PML elastic	2m 56s
Test run example 10 - homogeneous halfspace	3m 5s
Test run example 14 - waterlayered halfspace	3m 54s
Test run example 4 - small adjoint	4m 6s
Test run example 15 - tomographic model	2m 44s
Test run example 13 - PML acoustic	2m 15s
Test run example 5 - socal1D	3m 36s
Test run example 6 - socal1D 1d_socal	3m 22s
Test run example 7 - socal1D 1d_prem	3m 35s
Test run example 8 - socal1D 1d_cascadia	3m 33s
Test run example 9 - coupling FK	3m 31s
Test run example 16 - cavity	2m 36s
Test run example 17 - sep bathymetry	3m 27s
Test run example 18 - socal1D hdf5 i/o	3m 24s
	1h 45m 30s

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