Jupyter notebook: 1D decay-chain problem

In this pull request, we present several solutions to the decay-chain problem, as follows:

• an analytical solution;
• numerical solutions based on the operator splitting approach (time step size = 5 and 100 years);
• numerical solutions based on the global implicit approach (serial model and parallelized model with 4 and 8 processors);
• numerical solution obtained from a reference code (Dirichlet boundary condition needs to be re-examined);

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