Set default value of TSDataset.head method

[Mr-Geekman]

IMPORTANT: Please do not create a Pull Request without creating an issue first.

Before submitting (must do checklist)

• Did you read the contribution guide?
• Did you update the docs? We use Numpy format for all the methods and classes.
• Did you write any new necessary tests?
• Did you update the CHANGELOG?

Type of Change

• Examples / docs / tutorials / contributors update
• Bug fix (non-breaking change which fixes an issue)
• Improvement (non-breaking change which improves an existing feature)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Proposed Changes

Set default value of head method to 5 as it is in pandas.

Related Issue

#145.

Closing issues

Closes #145.

[codecov-commenter]

Codecov Report

Merging #170 (5609aec) into master (50f2074) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #170   +/-   ##
=======================================
  Coverage   87.38%   87.38%           
=======================================
  Files          73       73           
  Lines        3164     3164           
=======================================
  Hits         2765     2765           
  Misses        399      399           

Impacted Files	Coverage Δ
etna/datasets/tsdataset.py	86.43% <100.00%> (ø)

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