Refactor FDDE solvers

[ErikQQY]

Part of #70

[codecov]

Codecov Report

Attention: Patch coverage is 44.11765% with 114 lines in your changes are missing coverage. Please review.

Project coverage is 57.86%. Comparing base (6bb2f62) to head (042795d).
Report is 4 commits behind head on master.

Files	Patch %	Lines
src/delay/DelayABM.jl	0.00%	57 Missing ⚠️
src/delay/DelayPECE.jl	50.45%	55 Missing ⚠️
src/delay/product_integral.jl	93.54%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #102      +/-   ##
==========================================
- Coverage   62.89%   57.86%   -5.04%     
==========================================
  Files          34       34              
  Lines        3633     3681      +48     
==========================================
- Hits         2285     2130     -155     
- Misses       1348     1551     +203     

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