Resolve "Add a curve_fit operation and a (safe) lambda generator"

This MR adds the generate_lambda function, that invokes eval on highly specialised and sanitised strings:

• accepts only valid lambda definitions
• ... without a nested lambda
• highly restricts access to __builtins__
• prohibits some strings like __ and ;

This should be reasonably safe.

That operation is added to the operations database alongside curve_fit, which invokes scipy.optimize.curve_fit and requires a callable as first argument.

Can this MR be accepted?

• Implementation ready
• Tests added or adjusted
• Documentation extended or updated

• Code quality
  • Changes follow coding guidelines
  • Checked code coverage on new and adjusted code

• Ready for merging
  • Pipeline passes without warnings
  • History cleaned-up or squash option set
  • Changelog entry added
  • Version number bumped
  • Reviewed & approved by @herdeanu

Related issues

Closes #156 (closed)