Enhance Boundary: Add support for multiaxial prescribed value-argument

[adtzlr]

This PR closes #911.

Now it's possible to prescribe multiaxial values without manual broadcast+ravel required:

import felupe as fem
import numpy as np

mesh = fem.Cube(n=6)
region = fem.RegionHexahedron(mesh)
field = fem.FieldContainer([fem.Field(region, dim=3)])

value = np.array([1.0, 2.0, 3.0])
boundaries = dict(
    left=fem.Boundary(field[0], fx=0),
    right=fem.Boundary(field[0], fx=1, value=value),
)

dof0, dof1 = fem.dof.partition(field, boundaries)
ext0 = fem.dof.apply(field, boundaries, dof0)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.32%. Comparing base (43a7705) to head (b9a0d74).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #912   +/-   ##
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  Coverage   97.32%   97.32%           
=======================================
  Files         180      180           
  Lines        6424     6432    +8     
=======================================
+ Hits         6252     6260    +8     
  Misses        172      172           

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