[Solver] Explicit Modified Update Stress Last solver

[bodhinandach]

Describe the PR This PR quickly adds the explicit MUSL solver capability in our mpm_explicit solver. I thought it will be useful as the mpm::StressUpdate::MUSL is available, though never be used. Due to its considerably slower performance, if you think the MUSL option is not necessary, let's remove the MUSL option.

Additional context The MUSL option is basically similar to USL stress update, though require one more time momentum update right before recomputing strain_rate and strain for stress update. I ran the benchmark tests in both serial and parallel and both are ok! The necessity to do map_momentum_to_nodes and the compute_velocity one more time add a bit more computation time, so I am not sure if it is so useful. Here is the time comparison for the 3D hydrostatic column test in our benchmark test run in my local machine.

Stress Update	Runtime (s)
USF	512.5
USL	505.4
MUSL	560.2

[codecov[bot]]

Codecov Report

Merging #682 into develop will decrease coverage by 0.05%. The diff coverage is 68.29%.

@@             Coverage Diff             @@
##           develop     #682      +/-   ##
===========================================
- Coverage    96.66%   96.61%   -0.05%     
===========================================
  Files          123      123              
  Lines        25375    25412      +37     
===========================================
+ Hits         24527    24551      +24     
- Misses         848      861      +13     

Impacted Files	Coverage Δ
include/particles/particle.h	100.00% <ø> (ø)
include/particles/particle_base.h	100.00% <ø> (ø)
include/solvers/mpm_explicit.tcc	58.16% <27.78%> (-5.04%)	:arrow_down:
include/particles/particle.tcc	94.19% <100.00%> (+0.06%)	:arrow_up:
tests/mpm_explicit_usf_test.cc	100.00% <100.00%> (ø)
tests/mpm_explicit_usl_test.cc	100.00% <100.00%> (ø)
tests/particle_test.cc	99.87% <100.00%> (+<0.01%)	:arrow_up:

---

Continue to review full report at Codecov.

Legend - Click here to learn more Δ = absolute <relative> (impact), ø = not affected, ? = missing data Powered by Codecov. Last update 3b20da5...055b987. Read the comment docs.

[bodhinandach]

@kks32 I am okay. How you want to design or categorize them? I was thinking to call initialise node, compute_mass_momentum, and compute_velocity so that it was a set as the one done initially. However, the computational time becomes even slower as you do more mappings. Please feel free to commit your change @kks32.

[bodhinandach]

I am closing this PR and will reconsider the framework after looking at #685