__plotyy_axes__ misspell and __appdata__ isfield test

[Daniel J Sebald]

Ben,

Could you please look at this bug

https://savannah.gnu.org/bugs/index.php?37290

?  This boils down to rmfield() not checking for existence, but instead 
errors out so an "isfield" test must be done first.  The original report 
created a scenario for a path that apparently hadn't been tested before. 
 Verify that the patch does what was originally intended and update the 
development source if you have access.

Thanks,

Dan