Call funPart.dispData() in deltafun.dispData().
Call funPart.dispData() in deltafun.dispData().

This fixes a bug wherein calling disp() on a chebfun with a deltafun based on a
singfun would not print the singfun exponent information.  deltafun.dispData()
now retrieves all display data from the lower layers, as it needs to.

alshedivat/chebfun,asgeirbirkis/chebfun,ilovejs/chebfun