Encoding of plicas in **mens needs to be changed

[martha-thomae]

Hi @JRegimbal!

I just talked with @craigsapp and we have a minor issue with the plica encoding. The character p is already reserved in **mens to encode another mensural feature ("perfection," which is an important concept in mensural notation). To keep **mens backwards compatible, Craig changed the characters to encode plicas in **mens into k and K for plica up and plica down, respectively (see screenshot sent by Craig):

We can of course still use the keystrokes p/P to enter the plicas in the MP-Editor as @karend27 indicated, but they have to be mapped into the characters k/K in Humdrum (again, to don't break backwards compatibility of **mens).