Feature/contact/compute displacements

[thiagordonho]

Implement a function in the Particle class that maps the displacements from the material points to the nodal properties pool. Additionally, implement another function in the Node class to compute the nodal multimaterial displacements and separation vectors. Both of these functions are called from the MPMExplicit solve function within every computation step by iterating over all particles and all nodes, respectively. The following describes the rationale behind the map_multimaterial_displacements_to_nodes() function:

• Iterate over all the nodes in the cell this particle belongs to -- stored in nodes_.
• Compute the nodal displacement as mass_ * shapefn_[i] * displacement_ and update the nodal property at the node.

The following describes the rationale behind the compute_multimaterial_separation_vector() function:

• Iterate over all materials in the node -- stored in std::set material ids.
• Update the nodal displacements by dividing by the nodal-material mass.
• Calculate the displacement of the centre of mass at this node -- displacement_.
• Compute the separation vector as given in Nairn (2013) and assign it to nodal_properties_ (this computation is done in a separate iteration over all the materials, for the displacement of the centre of mass is necessary).

This PR is related to the RFC #636 . The calculation of the displacements and separation vectors for each node and material is expected with this PR. The separation vector is essential for the contact algorithm since it is used in a second verification of the contact between two adjacent materials.

[thiagordonho]

So far looks okay for me @thiagordonho. I added my suggestion and have one minor comment: will that be better if we somehow change the naming of displacement_ to something like contact_displacement_ to avoid confusion of nodal variable. I was thinking displacement_ is the integration of velocity_.

I think that is a very good idea! I will change all the related variables. Thank you for that and for the other remarks.

[codecov[bot]]

Codecov Report

Merging #649 into develop will decrease coverage by 0.00%. The diff coverage is 95.89%.

@@             Coverage Diff             @@
##           develop     #649      +/-   ##
===========================================
- Coverage    96.56%   96.56%   -0.00%     
===========================================
  Files          122      122              
  Lines        25247    25316      +69     
===========================================
+ Hits         24378    24444      +66     
- Misses         869      872       +3     

Impacted Files	Coverage Δ
include/node.h	100.00% <ø> (ø)
include/node_base.h	100.00% <ø> (ø)
include/particles/particle.h	100.00% <ø> (ø)
include/particles/particle_base.h	100.00% <ø> (ø)
include/solvers/mpm_explicit.tcc	62.75% <25.00%> (-1.25%)	:arrow_down:
include/mesh.tcc	84.06% <100.00%> (+0.07%)	:arrow_up:
include/node.tcc	95.67% <100.00%> (+0.34%)	:arrow_up:
include/particles/particle.tcc	94.27% <100.00%> (+0.08%)	:arrow_up:
tests/interface_test.cc	100.00% <100.00%> (ø)

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