Refactoring EncoderLaTeX

Refactoring EncoderLaTeX's encode() and decode() functions:

• Avoiding warnings if there is no real issue but also showing the input data close to the position where issues arose.
• Appending larger chunks of text where possible instead of inidividual characters. Hopefully more efficient, but not benchmarked.
• Minor code deduplication and refactoring as well as adding some comments.
• Adding tests in KBibTeXIOTest.