Add explicit nullness annotations

Description

Introduces strict nullity to the entire codebase.

SpotBugs checks for blatant violations and IntelliJ produces live feedback for potential bugs.

Changes

• Every package is annotated as non-null by default.
• Every (documented) nullable parameter is marked as such.

Additions

• JSR 305 support

Test and Review

Closes #130 (closed)

To be filled in by the reviewers

• All of the methods are commented to expectation

• The methods are tested to satisfaction

• There are no unnecessary files added to the PR

• The continuous integration has no problems with the PR

• The PR is filled in as requested e.g. labels added reviews requested

• The documentation is up-to-date

• All nullable parameters are marked as such