Add MeshContainer.as_vertex_mesh() and Field.from_mesh_container()

[adtzlr]

This PR adds a method to merge a mesh container into a vertex mesh. This can be used as a top-level mesh when dealing with multiple solid bodies with different element types. This is only useful if the mesh-container has been created with merge=True.

closes #937

import felupe as fem

container = fem.MeshContainer([fem.Rectangle()], merge=True)
mesh = container.as_vertex_mesh()
region = fem.RegionVertex(mesh)
field = fem.Field(region, dim=2).as_container()  # this is the top-level field

A more high-level approach is also available.

import felupe as fem

container = fem.MeshContainer([fem.Rectangle()], merge=True)
field = fem.Field.from_mesh_container(container).as_container()  # this is the top-level field

Further work for mixed-field solid-bodies is required. No, this works by adding the additional fields to the field container.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.20%. Comparing base (46b5093) to head (f223443).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #940      +/-   ##
==========================================
+ Coverage   97.19%   97.20%   +0.01%     
==========================================
  Files         181      182       +1     
  Lines        6524     6553      +29     
==========================================
+ Hits         6341     6370      +29     
  Misses        183      183              

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