Remove dependency on DualNumbers

[devmotion]

Fixes #57. Also to me it seemed this functionality is not used anywhere in the package, so it should not break existing code.

[codecov[bot]]

Codecov Report

Patch coverage: 85.71% and project coverage change: +0.06 :tada:

Comparison is base (98d6991) 77.81% compared to head (c2094fb) 77.87%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #59      +/-   ##
==========================================
+ Coverage   77.81%   77.87%   +0.06%     
==========================================
  Files           7        7              
  Lines        1109     1103       -6     
==========================================
- Hits          863      859       -4     
+ Misses        246      244       -2     

Impacted Files	Coverage Δ
src/HypergeometricFunctions.jl	100.00% <ø> (ø)
src/specialfunctions.jl	57.25% <85.71%> (+0.59%)	:arrow_up:

... and 1 file with indirect coverage changes

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