Initial solution on PIDE's

[Qfl3x]

See: #183

[codecov]

Codecov Report

Merging #216 (4099db4) into master (4d1b043) will increase coverage by 6.96%.
The diff coverage is 96.96%.

@@            Coverage Diff             @@
##           master     #216      +/-   ##
==========================================
+ Coverage   70.08%   77.05%   +6.96%     
==========================================
  Files          37       38       +1     
  Lines        1899     1931      +32     
==========================================
+ Hits         1331     1488     +157     
+ Misses        568      443     -125     

Impacted Files	Coverage Δ
...schemes/centered_difference/centered_difference.jl	100.00% <ø> (ø)
src/interface/solution/timedep.jl	85.29% <ø> (ø)
src/system_parsing/pde_system_transformation.jl	41.80% <88.88%> (+3.11%)	⬆️
src/discretization/discretize_vars.jl	86.66% <100.00%> (+13.33%)	⬆️
...discretization/generate_finite_difference_rules.jl	92.30% <100.00%> (+0.64%)	⬆️
...n/schemes/integral_expansion/integral_expansion.jl	100.00% <100.00%> (ø)
src/scalar_discretization.jl	75.60% <100.00%> (+1.25%)	⬆️
...ion/schemes/upwind_difference/upwind_difference.jl	88.40% <0.00%> (-2.64%)	⬇️
src/discretization/differential_discretizer.jl	97.43% <0.00%> (+2.56%)	⬆️
... and 10 more

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[xtalax]

I have added some tests for future cases such as Integral(Interval(xmin, xmax)) and Integral(Interval(a, b)), as well as your euler integrals. We can get to those when people need them, unless you want to have a go.

As a final touch, could you add a tutorial to the docs for integrals? An example like the PIDE you posted would work perfectly.

Thank you for your contribution! :)