[Initial condition] Add functions to initialize velocity and accelerations

[bodhinandach]

Describe the PR
This PR introduces a way to initialize the velocity and acceleration fields. To specify the inputs correctly, users need to specify the following in the .json file:

    "particles_velocities": {
      "type": "file",
      "location": "particle_velocities.txt"
    }

or

    "particles_accelerations": {
      "type": "file",
      "location": "particles_accelerations.txt"
    }

The .txt files should be located in the same working folder with input formats as follows for 3D:

0	0.000192735535593574	0.000192735535593574	0.000192735535593574
1	0.000191385852799918	0.000201731034032346	0.000201731034032346
2	0.000188889069065023	0.000210476391243882	0.000210476391243882
...
id      vx or ax                vy or ay                vz or az

For 2D, only three columns are necessary: id, vx/ax, vy/ay

Related Issues/PRs
N/A

Additional context
N/A

[codecov]

Codecov Report

Attention: Patch coverage is 18.39080% with 71 lines in your changes missing coverage. Please review.

Project coverage is 95.60%. Comparing base (d93ef19) to head (397cc8d).

Files with missing lines	Patch %	Lines
include/mesh/mesh.tcc	0.00%	26 Missing ⚠️
include/io/io_mesh_ascii.tcc	0.00%	23 Missing ⚠️
include/solvers/mpm_base.tcc	42.11%	22 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #105      +/-   ##
==========================================
- Coverage   95.73%   95.60%   -0.13%     
==========================================
  Files         244      244              
  Lines       51323    51410      +87     
==========================================
+ Hits        49130    49146      +16     
- Misses       2193     2264      +71     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.

[LDTalbot]

We should discuss input format for MP properties:

Some of our MP input files have ID while others do not (see recently-added wiki page).

• w/ ID: volumes, particles-cells
• w/o ID: particles (positions), stresses
• side note: mesh.txt also, of course, does not include node IDs in the first section with node positions.

Similarly input consistency notes:

• Particle count headers (e.g. stresses has this, while cells & volumes do not)
• JSON file input style (e.g. stresses is multiline, while cells & volumes are single)

The current PR has elements from different types (ID and no header like volumes and cells, but multiline input in JSON like stresses).

[LDTalbot]

#TODO

• Tests
• Particle sets option (if we keep 2-line JSON input format)

NOTES:

• We can alter volumes to use "read_particles_scalar_properties" in this PR (removing "read_particles_volumes")
• Should make separate PR for other proposed breaking changes re: particle input format consistency