Add slc option to Chain.get_draws and get_stats

[michaelosthege]

This PR adds a slc kwarg to the method signatures.
In a follow-up PR the implementations---particularly the ClickHouseChain._get_rows method---can be updated to make use of the slc information to select the data more efficiently (#50).

I added tests that assert that the slc functions in the same way as NumPy slicing the result manually.

Closes #47

[codecov-commenter]

Codecov Report

Merging #49 (bb42d58) into main (183407d) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main      #49   +/-   ##
=======================================
  Coverage   95.80%   95.80%           
=======================================
  Files           8        8           
  Lines         572      572           
=======================================
  Hits          548      548           
  Misses         24       24           

Impacted Files	Coverage Δ
mcbackend/__init__.py	71.42% <100.00%> (ø)
mcbackend/backends/clickhouse.py	100.00% <100.00%> (ø)
mcbackend/backends/numpy.py	100.00% <100.00%> (ø)
mcbackend/core.py	87.30% <100.00%> (ø)

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