add power-law slip weakening friction law (not including LTS) #203

Job	Run time
Check file changes	21s
Test Intel on ubuntu-20.04	5m 50s
Test on ubuntu-latest	1m 57s
Test on ubuntu-20.04	1m 55s
Test on Mac	8m 6s
Test run example 0 - make tests	0s
Test run example 2 - fault tpv5	0s
Test run example 3 - layered halfspace	0s
Test run example 1 - meshfem3D simple model	0s
Test run example 4 - small adjoint	0s
Test run example 5 - socal1D	0s
Test run example 6 - socal1D 1d_socal	0s
Test run example 7 - socal1D 1d_prem	0s
Test run example 10 - homogeneous halfspace	0s
Test run example 11 - poroelastic	0s
Test run example 12 - PML elastic	0s
Test run example 13 - PML acoustic	0s
Test run example 8 - socal1D 1d_cascadia	0s
Test run example 9 - coupling FK	0s
Test run example 14 - waterlayered halfspace	0s
Test run example 15 - tomographic model	0s
Test run example 16 - cavity	0s
Test run example 17 - sep bathymetry	0s
Test run example 18 - socal1D hdf5 i/o	0s
	18m 9s

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