Recreate Multinest in Pints

[ben18785]

Again to avoid dependency issues it should be possible (and informative) to recreate the Multinest sampling algorithm in Pints. See here.

[ben18785]

Note to self: add in importance nested sampling as an option. See: https://arxiv.org/pdf/1306.2144.pdf

[MichaelClerx]

Current notebook is here: https://github.com/pints-team/pints/blob/master/studies/multinest.ipynb (in master), as suggested in #480

[MichaelClerx]

For an alternative plan of adding a wrapper to existing software, see #161

[ben18785]

Have now recapitulated ellipsoidal results from paper (see f42dad0)

[ben18785]

Am adding to project for PINTS paper as nearly done.