Move sigma0 guessing code to hidden utility function

[MichaelClerx]

We have code that guesses a covariance matrix based on an input vector in a few different places, probably makes sense to write a single utility function that does this and call it where needed.

[fcooper8472]

Can you point me to these?

[MichaelClerx]

Here's one: https://github.com/pints-team/pints/blob/master/pints/_optimisers/init.py#L101-L135

And another: https://github.com/pints-team/pints/blob/master/pints/_mcmc/init.py#L87-L104

I think a few more are hidden in branches at the moment

They don't do exactly the same. It'd be good to have some utility function (or multiple?) with a name that makes sense, so that we can standardise this sort of thing.