Variable numbers of atoms between frames

In theory TNG files can have variable numbers of atoms between frames eg from a grand canonical simulation.  This is indicated by a flag in the TNG file  

```c++
typedef enum
{
    TNG_CONSTANT_N_ATOMS,
    TNG_VARIABLE_N_ATOMS
} tng_variable_n_atoms_flag;

```
While this is not supported upstream by MDA, we should look into whether introducing this is worth it for completeness sake.