Allow key input for ChordAnalyser

[titus-ong]

Currently, ChordAnalyser only accepts scales for its input, which is inconsistent with ChordRomanConverter which can accept keys. Allow ChordAnalyser to accepts keys as well in its analysing methods.

[titus-ong]

Will be included in #39. By default, the analyser should only accept keys rather than scales since that makes more sense musically (analyse a chord against the key signature, rather than against a scale).