Fix bug in unique ID specification for RF cavities. (#765)

* Update RFCavity.H and add examples.

* Update example files.

* [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci

* Implement a simpler solution.

* Detailed example as a separate PR.

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Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

src/particles/elements/RFCavity.H

@@ -144,7 +144,7 @@ namespace RFCavityData
           : Named(name),
             Thick(ds, nslice),
             Alignment(dx, dy, rotation_degree),
-            m_escale(escale), m_freq(freq), m_phase(phase), m_mapsteps(mapsteps)
+            m_escale(escale), m_freq(freq), m_phase(phase), m_mapsteps(mapsteps), m_id(RFCavityData::next_id)
         {
             // next created RF cavity has another id for its data
             RFCavityData::next_id++;