fix(CostL2): set min_size to 1

[deepcharles]

The min_size parameter was set to 2 for CostL2 and produced bad segmentations for small signals (see Issue #242).

Now it is set to 1 by default.

[deepcharles]

resolves #242

[codecov]

Codecov Report

Merging #255 (fbc6e4d) into master (939e7e2) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #255   +/-   ##
=======================================
  Coverage   98.77%   98.77%           
=======================================
  Files          40       40           
  Lines         978      978           
=======================================
  Hits          966      966           
  Misses         12       12           

Flag	Coverage Δ
unittests	98.77% <100.00%> (ø)

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Impacted Files	Coverage Δ
src/ruptures/costs/costl2.py	100.00% <100.00%> (ø)

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[oboulant]

I would add to the tests a special case for CostL2 where we explicitly check that the cost computed on a segment of size 1 returns 0.0 (and do not throw an NotEnoughPoints).