Rename PIIMTrap and PIIMRect

[ErikQQY]

Part of #70

[codecov]

Codecov Report

Merging #75 (6717074) into master (9d5ddf0) will decrease coverage by 0.03%.
The diff coverage is 83.33%.

@@            Coverage Diff             @@
##           master      #75      +/-   ##
==========================================
- Coverage   75.89%   75.86%   -0.03%     
==========================================
  Files          38       38              
  Lines        3609     3605       -4     
==========================================
- Hits         2739     2735       -4     
  Misses        870      870              

Impacted Files	Coverage Δ
src/FractionalDiffEq.jl	100.00% <ø> (ø)
src/fodesystem/bdf.jl	73.70% <70.00%> (-0.13%)	⬇️
src/fodesystem/newton_gregory.jl	74.64% <70.00%> (-0.13%)	⬇️
src/fodesystem/trapezoid.jl	76.52% <70.00%> (ø)
src/multitermsfode/implicit_pi_rectangle.jl	97.00% <100.00%> (-0.02%)	⬇️
src/multitermsfode/implicit_pi_trapezoid.jl	96.44% <100.00%> (-0.03%)	⬇️

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