Added a separate function for the influence function

[kaipartmann]

This does not solve #98 but makes it much easier to add a public API in the future. And for now, the function influence function can be redefined manually:

using Peridynamics

# redefinition of the internal function for OSBMaterial
@inline function Peridynamics.influence_function(::OSBMaterial, params::OSBPointParameters, L::Float64)
    return 1 + params.δ / L
end

# run a simulation with OSBMaterial here will use the redefined influence function

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.33%. Comparing base (bc81309) to head (abbd71f).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #143   +/-   ##
=======================================
  Coverage   90.32%   90.33%           
=======================================
  Files          43       43           
  Lines        3991     3995    +4     
=======================================
+ Hits         3605     3609    +4     
  Misses        386      386           

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