Implemented tridiag_solve for cubic spline

[pasabanov]

Types of changes

• Feature
• Documentation
• Refactoring, reformatting, cleanup
• Licensing

Related: #7 ALFI-lib/alfi-lib.github.io/pull/3 ALFI-lib/alfi-lib.github.io/pull/8

Description

1. Refactor:

   • Removed merge_coeffs function and replaced it with a loop.
   • Changed type for dX and dY from const Container<Number> to const auto.

2. Feature:

   • Added tridiag_solve_unstable function for use in cases where stability conditions are satisfied.
   • Added tridiag_solve function based on Add not-a-knot boundary conditions snsinfu/cxx-spline#2.
   • The function tridiag_solve is now used to compute coefficients for all splines, except for "Periodic", "Not-a-knot-start", "Not-a-knot-end" and "Semi-not-a-knot".
     It is used directly in "Not-a-knot", "Clamped", "Fixed-second" and "Fixed-third" and indirectly in "Natural" and "Parabolic-ends".

3. Documentation:

   • Added documentation for tridiag_solve_unstable and tridiag_solve.

4. Licensing:

   • Due to @snsinfu's code being under the BSL-1.0 license I had to place it somewhere in the repository. I put it in licenses/BSL-1.0 and added new file LICENSE where the temporary "default copyright law" license is stated for clarity.