Add tolerance for periodic face search.

[lijas]

I had a mesh which were not perfectly aligned, so I had to lower the search tolerance when matching the periodic faces. I just add the tolerance as a keyword for the user to set

[codecov-commenter]

Codecov Report

Patch coverage: 91.20% and project coverage change: -0.13 ⚠️

Comparison is base (2b6f9b1) 92.31% compared to head (f1743d0) 92.19%.

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #749      +/-   ##
==========================================
- Coverage   92.31%   92.19%   -0.13%     
==========================================
  Files          30       30              
  Lines        4527     4623      +96     
==========================================
+ Hits         4179     4262      +83     
- Misses        348      361      +13     

Impacted Files	Coverage Δ
src/PointEval/point_values.jl	97.36% <ø> (ø)
src/deprecations.jl	51.31% <0.00%> (-0.69%)	⬇️
src/Grid/grid.jl	92.59% <70.00%> (-0.15%)	⬇️
src/iterators.jl	84.37% <75.00%> (-3.96%)	⬇️
src/FEValues/face_values.jl	98.13% <77.77%> (-1.87%)	⬇️
src/interpolations.jl	96.53% <85.10%> (-1.36%)	⬇️
src/Dofs/DofHandler.jl	93.98% <91.83%> (+1.54%)	⬆️
ext/FerriteMetis.jl	100.00% <100.00%> (ø)
src/Dofs/ConstraintHandler.jl	95.97% <100.00%> (ø)
src/Dofs/DofRenumbering.jl	91.30% <100.00%> (ø)
... and 7 more

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