data encapsulation and integration point module

- xnode module, material module, isotropic type module and a newly added
integration point module are modified to hide components of the derived
types (except the iso modulus, strength and toughness types which are
public, because their intrinsic constructors are needed; plus, they are
not directly used in any external procedure, they're always part of the
iso type, so them being public does no harm)

- an export procedure is added to the above modules to facilitate the
extraction of information from the objects

- the above modified modules have derived type's components initialized
(zeroed or set .false.) at definition, in addition to the empty
procedure; safer is always better

-next step, finish the tri element module, and the associated procedures
in toolkit module