add a transport problem to the dg script

See merge request !78

# This is the 1st commit message:

add a transport problem to the dg script

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# minor changes - still not stable

add a script demonstrating the computation of eigenvalues/eigenfunctions

See merge request !76

Contribution by Philip Herbert

make some changes to dirichlet eigenvalue problem

improve the index a bit

See merge request !80

- move link to below search box (and to end of toc)
- use 'pair:' to get multiple index entries

Remove description of 'cppFunction' from 'concepts.py'.

See merge request !81

It is moved into a new script added to 'Using C++ Code Snippets'.
That concept was too advanced for 'concepts.py'.

improve the toc by adding rst file for each chapter

See merge request !77