Add fermata attribute for mensural notes

[martha-thomae]

The fermata attribute (found in the att.fermataPresent class) is only available for notes in CMN.

In this PR, I add the missing class att.note.anl.mensural (the logical, gestural, and visual corresponding classes already exist), and make it a member of att.fermataPresent, similar to how att.note.anl.cmn is a member of att.fermataPresent. Then include the newly introduced att.note.anl.mensural class in the classes of att.note.anl.

[craigsapp]

I do not have a problem with allowing <fermata>, but note that it is often called corona in mensural music. with the link saying:

It is similar to the fermata in modern notation.

Perhaps there is no functional difference between them, in which case using fermata as the name for the mensural version is no problem. (I am not aware of any difference at all, at least in the music of Josquin).

Perhaps some mensuralists could comment on if there is any real difference between them?

[craigsapp]

Jesse Rodin cannot think of any difference between a fermata and a corona, so it should be safe to assume that they are equivalent.

[martha-thomae]

Hi @craigsapp! Thank you for checking on this :)

[kepper]

sounds good to me