pp_ser: insertion of USE statement can cause problem

[clementval]

Currently, pp_ser insert USEstatement just before it finds IMPLICIT NONE

It can cause problem in the following example:

MODULE dummy

CONTAINS

SUBROUTINE a()
  IMPLICIT NONE
  !$ser data ...
END SUBROUTINE a

SUBROUTINE b()
  IMPLICIT NONE
  !$ser data ...
END SUBROUTINE b

END MODULE

In this case, the USEstatement will be added just before the IMPLICIT NONEin subroutine a()but nothing will be added in subroutine b()

The USEstatement should always be added at the beginning of the MODULEif it is a module

[clementval]

Fix for this issue should fix the issue #5 as well

[havogt]

@clementval Is it fixed?