[Feature] VTK output vars

[bodhinandach]

Describe the PR
This PR solves issue #594 to output variables from JSON with unknown type. A new map to link the string specified in JSON and a new VariableType class is created. Also, this removes the default output variables of displacements, velocities, stresses, and strains.

Every time a user wants to output a variable, just add the string and type to the list specified in src/mpm.cc and at the initialise() function in particle.tcc for which variable to return using lambda function.

The JSON list "vtk" is ready back to be used.

Related Issues/PRs
Issue #594

Additional context

1. The current design work only for vector and tensor variables in particle, the scalar part is still ongoing development. (Done ✔️)
2. @kks32 Maybe we might want to use the flat map that you test to have a fast retrieval. (Done using tsl_map ✔️)

[codecov]

Codecov Report

Merging #684 into develop will decrease coverage by 0.01%.
The diff coverage is 95.59%.

@@             Coverage Diff             @@
##           develop     #684      +/-   ##
===========================================
- Coverage    96.68%   96.67%   -0.01%     
===========================================
  Files          123      123              
  Lines        25391    25560     +169     
===========================================
+ Hits         24549    24710     +161     
- Misses         842      850       +8     

Impacted Files	Coverage Δ
include/mesh.h	100.00% <ø> (ø)
include/particles/particle_base.h	100.00% <ø> (ø)
include/solvers/mpm_base.h	0.00% <ø> (ø)
tests/io/write_mesh_particles.cc	87.27% <ø> (ø)
tests/io/write_mesh_particles_unitcell.cc	87.26% <0.00%> (-0.41%)	⬇️
include/solvers/mpm_base.tcc	76.09% <87.18%> (+0.48%)	⬆️
include/particles/particle.tcc	93.93% <91.67%> (-0.20%)	⬇️
include/mesh.tcc	84.13% <100.00%> (+0.23%)	⬆️
include/particles/particle.h	100.00% <100.00%> (ø)
tests/mesh_test_2d.cc	98.46% <100.00%> (+0.02%)	⬆️
... and 2 more

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