Fix thinning and burning in list sampler

[joezuntz]

Addresses issue #122

Also I spotted a couple of other issues along the way that this fixes:

• The successful run count was being reported incorrectly
• The Sampler.get_sampler function was not working.
• Added the output classes to be imported into main namespace

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 92.85714% with 3 lines in your changes are missing coverage. Please review.

Project coverage is 61.82%. Comparing base (5f92934) to head (57ef8cc).

Files	Patch %	Lines
cosmosis/samplers/minuit/minuit_sampler.py	33.33%	2 Missing :warning:
cosmosis/samplers/list/list_sampler.py	90.00%	1 Missing :warning:

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #127 +/- ## ========================================== + Coverage 61.17% 61.82% +0.64% ========================================== Files 115 115 Lines 11942 11973 +31 ========================================== + Hits 7306 7402 +96 + Misses 4636 4571 -65 ```

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