Add section comments on ein-cell.el

I will need to reorder functions, but it's better than nothing.

ein-cell.el

@@ -34,6 +34,8 @@
 (require 'ein-utils)
 (require 'ein-node)
 
+
+;;; EIEIO related utils
 
 (defmacro ein:oset-if-empty (obj slot value)
   `(unless (and (slot-boundp ,obj ,slot) (oref ,obj ,slot))
@@ -43,6 +45,8 @@
   `(when (slot-boundp ,obj ,slot)
      (oref ,obj ,slot)))
 
+
+:;; Cell classes
 
 (defclass ein:basecell ()
   ((cell-type :initarg :cell-type :type string)
@@ -80,6 +84,9 @@
 (defclass ein:rstcell (ein:textcell)
   ((cell-type :initarg :cell-type :initform "rst")))
 
+
+;;; Cell factory
+
 (defun ein:cell-class-from-type (type)
   (ein:case-equal type
     (("code") 'ein:codecell)
@@ -107,6 +114,9 @@
       (oset cell :input it))
   cell)
 
+
+;;; Getter/setter
+
 (defmethod ein:cell-num-outputs ((cell ein:codecell))
   (length (oref cell :outputs)))
 
@@ -157,6 +167,9 @@ A specific node can be specified using optional ARGS."
   (append (call-next-method)
           (ein:cell-element-get cell :output)))
 
+
+;; EWOC
+
 (defun ein:cell-make-element (make-node num-outputs)
   (list
    :prompt (funcall make-node 'prompt)
@@ -291,6 +304,9 @@ A specific node can be specified using optional ARGS."
   ;; Skip the newline
   (forward-char))
 
+
+;; Data manipulation
+
 (defun ein:cell-clear-output (cell stdout stderr other)
   ;; codecell.js in IPytohn implements it using timeout and callback.
   ;; As it is unclear why timeout is needed, just clear output