Rename all cxx files and usages to cpp

Created by: rerpha

Description of work

Renamed all .cxx files to *.cpp

Issue

Closes #185 (closed)

Acceptance Criteria

Run filewriter as usual, run unit tests and check they pass

Unit Tests

No unit tests modified besides documentation

Other

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Code Review (To be filled in by the reviewer only)

• Is the code of an acceptable quality?
• Do the changes function as described and is it robust?

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Nominate for Group Code Review (Anyone can nominate it)

Indicate if you think the code should be reviewed in a Thursday code review session.

• Recommend for group code review

Also, nominate it on the code_review Slack channel (does someone want to automate this?).