Added new parameter USE_OTHER_TIME_FUNCTION

[homnath]

In my view, the best way to reconcile source time functions for CMTSOLUTION and FORCESOLUTION is to create a new common parameter for both CMTSOLUTION and FORCESOLUTION similar to:
SOURCE_TIME_FUNCTION = gaussian
and get rid of USE_RICKER_TIME_FUNCTION from the Par_file and "source time function" from the FORCESOLUTION.

However, to minimise the code changes, I have added a new parameter, USE_OTHER_TIME_FUNCTION, for now. Currently, it is implemented only for viscoelastic simulations, and the other available functions are Brune and Smooth Brune.

[codecov]

Codecov Report

Attention: 69 lines in your changes are missing coverage. Please review.

Comparison is base (0321f61) 51.47% compared to head (c90b2f7) 39.41%.

❗ Current head c90b2f7 differs from pull request most recent head 69287a5. Consider uploading reports for the commit 69287a5 to get more accurate results

Files	Patch %	Lines
src/specfem3D/locate_source.F90	8.57%	32 Missing ⚠️
src/specfem3D/setup_sources_receivers.f90	30.00%	14 Missing ⚠️
src/specfem3D/compute_add_sources_viscoelastic.F90	7.14%	13 Missing ⚠️
src/specfem3D/get_cmt.f90	37.50%	5 Missing ⚠️
src/shared/read_parameter_file.F90	50.00%	3 Missing ⚠️
...roblem_for_model/input_output/input_output_mod.f90	87.50%	1 Missing ⚠️
src/shared/count_number_of_sources.f90	50.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##            devel    #1659       +/-   ##
===========================================
- Coverage   51.47%   39.41%   -12.06%     
===========================================
  Files         266      266               
  Lines       60241    60331       +90     
===========================================
- Hits        31009    23780     -7229     
- Misses      29232    36551     +7319     

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