Add TwoPointSecondOrderBVProblem constructor

[ErikQQY]

Similar to TwoPointBVProblem, this PR adds the TwoPointSecondOrderBVProblem constructor.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 19 lines in your changes are missing coverage. Please review.

Project coverage is 31.72%. Comparing base (8e7c260) to head (ae3ac8c).

❗ Current head ae3ac8c differs from pull request most recent head b5d9ef7. Consider uploading reports for the commit b5d9ef7 to get more accurate results

Files	Patch %	Lines
src/problems/bvp_problems.jl	0.00%	19 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #676      +/-   ##
==========================================
- Coverage   31.85%   31.72%   -0.14%     
==========================================
  Files          55       55              
  Lines        4486     4505      +19     
==========================================
  Hits         1429     1429              
- Misses       3057     3076      +19     

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