Add note saying we won't have discrete parameters in PINTS

[ben18785]

To begin with we should probably state that PINTS does not allow discrete parameters.

Later on, we should probably implement some discrete samplers - for some of the samplers, these exist and are straightforward.

However should encourage users to marginalise out their discrete variables when possible due to Rao-Blackwellisation.

[ben18785]

List of potential discrete samplers:

• Continuous relaxation HMC
• Metropolis
• Population
• Nested(?)

[MichaelClerx]

Just decided in meeting that current solution is:

• [ ] To begin with we should probably state that PINTS does not allow discrete parameters.

[MichaelClerx]

@ben18785 where do you think this note should go? Somewhere in README? Somewhere in the docs?

[ben18785]

Yep, in the README -- I can't think where it would go in the docs (other than in lots of places!)