Check the RAM occupied by SOAP calculations

Stop the calculations if the SOAP array size will be large compared to the available RAM. 

Something like 

```
dtype = np.float32  # or np.float64

mem_bytes = n_atoms * n_frames * soap_dim * np.dtype(dtype).itemsize
mem_MB = mem_bytes / 1024**2
mem_GB = mem_bytes / 1024**3
``` 
?