Fix use of wrong variable in @chebfun/constructor.m.

As Nick Hale points out, this "op" needs to be "opk".  This mistake was causing
problems when constructing from a cell array of function handles with
breakpoint detection enabled.

This closes #1151, at least in that it fixes the error/crash reported there.
Use eps instead of epslevel(f) when setting htol in chebfun.roots.

Using epslevel to construct this tolerance doesn't really make sense.  As Alex
Townsend points out, the subdivision process perturbs roots by eps*hscale(f),
not epslevel(f)*hscale(f).  Moreover, a function's epslevel tells us (sort of)
how accurate it is relative to its _vertical_ scale and has essentially nothing
to do with its _horizontal_ scale.

This closes #1146, which showed that setting htol using epslevel(f) instead of
eps can cause us to miss roots near breakpoints if the function is unresolved.
Merge branch 'fix-roots-htol' into development