added a reference to EM paper Morency gji 2020

[cmorency1]

Update manual with a section 4.5 describing electromagnetic wave propagation simulations.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 54.63%. Comparing base (baeb71d) to head (76f4636).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## devel #1224 +/- ## ======================================= Coverage 54.63% 54.63% ======================================= Files 164 164 Lines 30233 30233 ======================================= Hits 16519 16519 Misses 13714 13714 ```

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[danielpeter]

all good to go :)