Dram simplification (and addressing bug)

[ben18785]

• Simplifies DRAM so it only takes 2 proposal kernels (I think it was overkill to allow more before)
• In doing so, sorts bug highlighted in #1337
• Notebook updated to reflect new functioning

To do:

• [ ] Run functional tests and illustrate convergence in each

For the review, it'd be good if the persons charged with this task could look at the set_hyperparameters function. At the moment, I'm not sure whether this is optimal since one of the hyperparameters is a list.

[codecov[bot]]

Codecov Report

Merging #1339 (113869c) into master (2afaee9) will not change coverage. The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master     #1339   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           95        95           
  Lines         9282      9263   -19     
=========================================
- Hits          9282      9263   -19     

Impacted Files	Coverage Δ
pints/_mcmc/_dram_ac.py	100.00% <100.00%> (ø)

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