Move preamble and title page to their own files
- Moved preamble from main.tex → preamble.tex.
- Moved title page from main.tex → titlepage.tex.
- Automated primary language selection on title page via language commands
defined in the file tauthesis.cls. Now a user no longer needs to modify
these themselves, which should provide for a better user experience.
- Added metadata commands \myyear, \mymonth and \myday in main.tex.
- Changed the command invocation \documentclass{tauthesis} →
\documentclass[finnish]{tauthesis}, to make it clear how the language is
selected.