Refactor multi-terms FODE solvers

[ErikQQY]

Part of #70

[codecov]

Codecov Report

Attention: Patch coverage is 87.62887% with 48 lines in your changes are missing coverage. Please review.

Project coverage is 62.89%. Comparing base (9709d83) to head (6bb2f62).
Report is 3 commits behind head on master.

Files	Patch %	Lines
src/multitermsfode/matrix.jl	7.14%	26 Missing ⚠️
src/multitermsfode/implicit_pi_pece.jl	89.89%	10 Missing ⚠️
src/multitermsfode/implicit_pi_trapezoid.jl	95.74%	4 Missing ⚠️
src/multitermsfode/explicit_pi.jl	95.77%	3 Missing ⚠️
src/multitermsfode/implicit_pi_rectangle.jl	96.42%	3 Missing ⚠️
src/types/problems.jl	71.42%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #101      +/-   ##
==========================================
- Coverage   64.59%   62.89%   -1.70%     
==========================================
  Files          34       34              
  Lines        3587     3633      +46     
==========================================
- Hits         2317     2285      -32     
- Misses       1270     1348      +78     

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