FODE solvers overhaul

[ErikQQY]

#70

[codecov]

Codecov Report

Attention: Patch coverage is 17.45690% with 383 lines in your changes are missing coverage. Please review.

Project coverage is 58.05%. Comparing base (fe173b7) to head (97fca67).

Files	Patch %	Lines
src/fode/implicit_pi_rectangle.jl	0.00%	185 Missing ⚠️
src/fode/implicit_pi_trapzoid.jl	0.00%	185 Missing ⚠️
src/types/problems.jl	0.00%	7 Missing ⚠️
src/fode/bdf.jl	76.92%	3 Missing ⚠️
src/multitermsfode/implicit_pi_pece.jl	92.30%	1 Missing ⚠️
src/multitermsfode/implicit_pi_rectangle.jl	92.30%	1 Missing ⚠️
src/multitermsfode/implicit_pi_trapezoid.jl	92.30%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #104      +/-   ##
==========================================
- Coverage   65.48%   58.05%   -7.44%     
==========================================
  Files          34       36       +2     
  Lines        3685     4062     +377     
==========================================
- Hits         2413     2358      -55     
- Misses       1272     1704     +432     

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