add %class %dtype methods

h5fortran can now query data "%class" e.g. integer, float, string as well as the native datatype "%dtype"

Dataset "dname" data class (i.e. integer, float, string, ...)

integer :: class
!! H5T_INTEGER_F, H5T_FLOAT_F, H5T_STRING_F
class = h%class(dname)
character(*), intent(in) :: dname

Dataset "dname" datatype

integer(HID_T) :: dtype
!! H5T_NATIVE_REAL, H5T_NATIVE_DOUBLE, H5T_NATIVE_INTEGER, H5T_NATIVE_CHARACTER, H5T_STD_I64LE
dtype = h%dtype(dname)
character(*), intent(in) :: dname