ENH: Initialize PolyLineCell with 2 points

[PranjalSahu]

PolyLine Cell should have at least two points so inserting those points in the default constructor.
For code re-use added InitializePoints method that is used in constructor and ClearPoints method.

PR Checklist

• No API changes were made (or the changes have been approved)
• No major design changes were made (or the changes have been approved)
• Added test (or behavior not changed)
• Updated API documentation (or API not changed)
• Added license to new files (if any)
• Added Python wrapping to new files (if any) as described in ITK Software Guide Section 9.5
• Added ITK examples for all new major features (if any)

Refer to the ITK Software Guide for
further development details if necessary.