Update file headers

## Issue Description

Type: *bug report*

### Describe what happened (or what feature you want)
Files headers must be applied and updated on all source files according to the terms of the Apache license.

### Describe what you expected to happen
All production and test sources should have their file headers up to date.

### Tell us your environment

Apache Maven 3.6.0 (97c98ec64a1fdfee7767ce5ffb20918da4f719f3; 2018-10-24T20:41:47+02:00)
Maven home: /Users/aalmiray/.sdkman/candidates/maven/current
Java version: 1.8.0_191, vendor: Oracle Corporation, runtime: /Library/Java/JavaVirtualMachines/jdk1.8.0_191.jdk/Contents/Home/jre
Default locale: en_CH, platform encoding: UTF-8
OS name: "mac os x", version: "10.13.6", arch: "x86_64", family: "mac"

Start by auditing the repository's production and test source files for missing or outdated Apache license headers, using the Apache license terms as the reference. Done means every production and test source has an up-to-date compliant header; the issue does not name specific files or a validation command.

Written by the indexing model from the issue text.