[Fortran] replace pointer with allocatable

In Fortran, it is possible to create an array with
```fortran
real*8, dimension(:), pointer :: vec
allocate(vec(100))
```
the effect of this code is to allocate an array, and associate the pointer `vec` to it. Unfortunately, since it is a pointer, it will not be automatically deallocated when it goes out of scope, and therefore could cause a **memory leak**. A safer way of allocating an array is
```fortran
real*8, dimension(:), allocatable :: vec2
allocate(vec2(100))
```

Unfortunately, it is not just a matter of replacing `pointer` with `allocatable`, since there are also other steps.

First of all, we must distinguish, which arrays can become allocatable from the ones that must remain pointers. Consider the following example
```fortran
real*8, dimension(:), pointer :: vec
real*8, dimension(:), pointer :: p_vec
allocate(vec(100))
p_vec => vec(20:50)
```
In this case, only `vec` should become an allocatable.

Since it is good practice to nullify pointers (otherwise their status is undefined), there might be a bunch of `vec=>null()` and `nullify(vec)`, which must be deleted.

Next, the code might contain several `if (associated(vec))`, which must be replaced with `if (allocated(vec))`.

The last thing to address is that when a pointer is associated to an allocatable, that allocatable needs the `target` attribute. For example
```fortran
real*8, dimension(:), allocatable, target :: vec2
real*8, dimension(:), pointer :: p_vec
allocate(vec2(100))
p_vec => vec2(20:50)
```

A slightly more complex case is
```fortran
type t_special
real*8 :: a
real*8, dimension(:), allocatable :: b
end t_special

type(t_special), dimension(:), allocatable, target :: c
type(t_special), dimension(:), pointer :: p_c

allocate(c(15))
p_c => c
```
Note that in this case the `target` attribute is applied to `c`, and not to the fields of the derived type.