Move design logic across to Conveyance class

[ScottMcCormack]

• Move design logic from test class across to `conveyance.Conveyance.
• Move nested functions into Class

[codecov-commenter]

Codecov Report

Merging #9 (5deec78) into main (748e44d) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##              main        #9   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            5         6    +1     
  Lines          215       220    +5     
  Branches         6         6           
=========================================
+ Hits           215       220    +5     

Impacted Files	Coverage Δ
src/conveyance/conveyor_resistances.py	100.00% <ø> (ø)
src/conveyance/__init__.py	100.00% <100.00%> (ø)
src/conveyance/conveyance.py	100.00% <100.00%> (ø)
tests/test_flat_conveyor.py	100.00% <100.00%> (ø)

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