Add densitytensor notebook

[SamDuffield]

Added example notebook demonstrating densitytensor simulator

[erinaldiq]

Great addition to the examples.

One sentence I would clarify is

Now let's sample some measurements from the true $p$

It sounds strange to sample from $p$. What we actually do is sample from both the noiseless and the noisy circuit with noise parameters $p$.

[SamDuffield]

Great addition to the examples.

One sentence I would clarify is

Now let's sample some measurements from the true $p$

It sounds strange to sample from p. What we actually do is sample from both the noiseless and the noisy circuit with noise parameters p.

Good point! Yeah really what we sample are "noisy measurements from the circuit with noise parameter $p$"