mark identity() function for removal

[GavinHuttley]

What should be changed

cogent3.util.misc.identity()

Why

Not used in cogent3.

How much notice

6 months notice.

Guidelines for deprecation

Checking no internal uses will be tricky numpy and scipy also have functions called identity().

As this is a function that will be removed, see dev guidelines for deprecation of code. Indicate the limiting version (for when the old function will be removed) as .<PR month + x> where x is defined above.

[sharonli126]

I would like to do this issue

[GavinHuttley]

awesome! let me know if you run into any difficulties.

[sharonli126]

Hi Gavin, I have fixed the issue and synced the change. Please have a look. thanks.