Package IFDIFF as community toolbox

[Schlevidon]

Changes

Implementing MATLAB Toolbox Best Practices

• Introduce toolbox subfolder which contains all files that should be distributed to users.
  • At the top-level: functions that are directly accessed by users (setting up IFDIFF, prepare/solve ODE, sensitivities)
  • In sub-folders examples and doc: Example problems and documentation.
  • In sub-folder internal: All other implementation details categorized into sub-folders by functionality. The idea is that a user should never have to actively examine or interact with files in this folder.
• Use standardized folder naming: lower camelCase.
• Add toolbox packaging file.

Other changes

• Improved path handling in tests using fixtures (should prevent minor annoyance where preprocessed functions generated in tests would sometimes end up in two different locations due to incorrect handling of directory changes).
• Fix critical error when exporting model functions during sensitivity computation for a RHS that contains helper functions.
• Removed some outdated test files.
• Add wrapper for initIFDIFF in repo root, so that the actual function in toolbox/initIFDIFF.m is still callable without changing directory first.