Fixed arrayAddons.hpp for gcc 10.1 (#972)
Fixed arrayAddons.hpp for gcc 10.1 (#972)

## Technical Description

Fixes incorrect usage of `size_t` in `arrayAddons.hpp`

### Resolves / Enhances

None.

### Example

Compiliation with gcc 10.1.

### Testing

Was tested locally with gcc 10.1.

### Dependencies

None

### Commets

We could also include `stddef.h` instead and then `size_t` will be defined in the _global_ namespace.
MeteoSwiss-APN/dawn,MeteoSwiss-APN/dawn,MeteoSwiss-APN/dawn