Improve function get_rotation_matrix

[ahojukka5]

• Added docstring explaining the principles of constructing orthonormal local base for defining beam cross-section.
• Now function throws an exception with a clear error message if user tries to construct rotation matrix where tangent is parallel to first beam section axis n1.
• This commit closes PR #10.

[coveralls]

Pull Request Test Coverage Report for Build 56

• 4 of 4 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.01%) to 98.851%

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Totals
Change from base Build 51:	0.01%
Covered Lines:	258
Relevant Lines:	261

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💛 - Coveralls