LSI correlator: plot correlation function on-the-go

(@smk78 edit of @rerpha text)

As a user of the LSi correlator, I would like to be able to see my correlation function as the data is gathered. This is useful i) when setting up/aligning for a measurement (involving lots of quick measurement-tweak cycles) and, ii) for judging the acquired statistics as the measurement proceeds (else one must grossly over-estimate the measurement duration).

This 'live view' functionality is available in the vendor software, but despite being an ethernet device the LSi correlator is unable to talk to both the vendor software and Ibex at the same time. Indeed the correlator IOC must be stopped in order for the vendor software to run. This means at present a user must do multiple measurements, each time allowing the correlator data to be written to file, and then inspecting the data only once each measurement has finished, which is very time-consuming.

The correlation function plot looks like this

This data is stored in a numpy array.

In the first instance we should investigate whether or not this can be made available at run-time.

### Acceptance Criteria
_What is the acceptance criteria?_
- [ ] A plot showing the correlation function is added to the lsi correlator server, and then shown on a tab on the OPI.

### Extra Information
Matplotlib may be best for this use case.

https://lsinstruments.ch/en/theory/dynamic-light-scattering-dls/introduction

## How to Test
_verbose instructions for reviewer to test changes
(**Add before making a PR**)_