Provide method to return the count of contained items in the CUDS containers

[itziakos]

A set of CUBA keywords will be added for the low level CUDS items.

POINT
PARTICLE
BOND
EDGE
FACE
CELL
NODE

New method in CUDS containers

  def count_of(self, item_type):
      """ Return the count of item_type in the container.

      Parameter
      ---------
      item_type : CUBA
          The CUBA enum of the type of the items to return the count of.         

      Returns
      -------
      count : int
          The number of items of item_type in the container.

      Raises
      ------
      ValueError :
          If the type of the item is not supported in the current container. 

      """

tasks:

• update the KEYWords yml and regenerate the CUBA enum
• update the uml diagrams
• Augment test template
• update the abstract classes
• update the concrete classes
• update the hdf5 back classes