Allow choice of tau2 estimation method

[andrjohns]

An interesting additional feature could be the ability to use REML estimation to obtain the tau2 term, which could also allow for additional levels of variance terms for inflating the control limits.

A possible workflow could be:

1. User calls tau2_method="REML" in the funnel_plot call (where the tau2_method option would default to DL)
2. Construct unadjusted z-scores
3. Truncate data based on unadjusted z-scores
4. Pass truncated data to metafor::rma.mv, using the transformed outcome and standard errors
5. Extract estimated variance

Alternatively, metafor's escalc function could be used directly for constructing the standard errors.

Let me know what you think, happy to work this up and open a PR

[chrismainey]

That's a really good option. I'll think on that. Happy to collaborate on it, or receive a PR. Thanks!