WIP: Transpose

[earboxer]

Based on code and discussion from #1189.

[lpugin]

Could you make the PR to develop-transpose for now? Also, there is an error with AppVeyor. Any thoughts?

[earboxer]

@lpugin I changed it to target develop-transpose, and also added #include <cctype>, which should allow isdigit to work.

Do we need to change appveyor.yml so that it runs on develop-transpose?

[craigsapp]

Should I CamelCase the functions in the Transpose class? Perhaps not necessary since the class is not directly related to MEI functionality?

[lpugin]

Should I CamelCase the functions in the Transpose class? Perhaps not necessary since the class is not directly related to MEI functionality?

Yes please. (This is a Verovio coding guideline)