Sig kernel computations halt if n_seq = 1

Running the following code results in the jupyter kernel crashing:

```python
n_levels = 5
static_kernel = ksig.static.kernels.RBFKernel() 
sig_kernel = ksig.kernels.SignatureKernel(n_levels, static_kernel=static_kernel)
N, T, d = 1, 50, 5 
n_seq = 1
X = np.random.randn(N, T, d)
Y = np.random.randn(n_seq, T, d)
sig_kernel(X, Y)
```

This only happens if both N and n_seq is 1.