Verbatim copy of MakeMaskFromDensity
new threshold generic + threshold.im3d method
test new threshold function
update @family directives
tag threshold.im3d for export

* since method was not properly exported package check() failed
Teach im3d() to accept an im3d object to supply attributes

* this can be x itself so that im3d(x) returns x 
  (minus any non-standard attributes)
correct and simplify threshold.im3d implementation

* depends on new im3d() constructor