Adds linewidth param to GenericMaterial.

[ferrolho]

Hey, @rdeits! I have added linewidth to GenericMaterial. Let me know if you think linewidth is not generic enough and, in that case, whether a new subtype of AbstractMaterial should be added. Thanks!

[codecov-io]

Codecov Report

Merging #103 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           master    #103   +/-   ##
======================================
  Coverage    75.2%   75.2%           
======================================
  Files          13      13           
  Lines         363     363           
======================================
  Hits          273     273           
  Misses         90      90

Impacted Files	Coverage Δ
src/lowering.jl	81.42% <ø> (ø)	⬆️
src/objects.jl	100% <ø> (ø)	⬆️

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[rdeits]

Thanks!