Add missing Boundary.c in CactusWave/WaveToyExtra (einsteintoolkit/Cactus#3)

This PR adds the missing source file Boundary.c to complete the CactusWave/WaveToyExtra thorn implementation.

Implement the full functionality of WaveToyExtra

Please implement the full functionality of `WaveToyExtra` by finishing the code in `Boundary.c`.

## Thorn Information:
- Name: WaveToyExtra
- Target file: Boundary.c

## Interface Definition in interface.ccl:

## Schedule Definition in schedule.ccl:

## Parameters Definition in param.ccl:

## Configuration Definition in configuration.ccl:

## Documentation Context in readme.md and doc/documentation.tex

## Related Code Context in src folder

diff --git a/all_tests.txt b/all_tests.txt
new file mode 100644
index 0000000..0000000
--- /dev/null
+++ b/all_tests.txt
@@ -0,0 +1,1 @@
+test_custom