Fix logic in read_iasing for deriving scan positon (1-56) from field-of-view (1-224).

[wx20jjung]

There is a logic error in deriving scan position from field-of-view. When the value of mod(istep,4) = 0, the position is off by 4. An example would be ipos = 20 and should be ipos = 24.

Here is the current code:
ifor = (ifov-1) / 16 ! Determine field-of-regard
istep = ifov - ((ifor) * 16) ! Determine field-of-view within field-of-regard
ipos = (ifor * 4) + mod(istep,4) ! Determine position of field-of-view within scan line