Implement forecast function

[ken-swyfft]

This is my first PR for NPOI, and I'm still very much a newbie when it comes to the codebase. But this was a function that we needed, and it seemed like this was the easiest way to get it done.

Please let me know if there's anything I could/should be doing differently or better, or if there's anything I missed.

Copilot Description
This pull request introduces changes to the codebase to implement the Excel FORECAST function in the NPOI.SS.Formula.Functions namespace. The most significant changes include the addition of the Forecast class, a modification to the ProduceFunctions method in the FunctionEval class to use this new Forecast class, and the addition of unit tests to verify the functionality of the Forecast class.

Here are the top five most important changes:

Codebase expansion:

• main/SS/Formula/Functions/Forecast.cs: Added the Forecast class which represents the Excel FORECAST function. This function predicts a future value along a linear trend line based on existing historical data. The class includes methods for converting ValueEval objects to numeric arrays and for creating ValueVectors.

Codebase modification:

• main/SS/Formula/Eval/FunctionEval.cs: Modified the ProduceFunctions method to use the new Forecast class instead of the NotImplementedFunction for the FORECAST function.

Codebase configuration:

• build/_build.csproj.DotSettings: Added user rules for C# naming conventions and updated the settings migration to reflect these changes. [1] [2]

Testing:

• testcases/main/SS/Formula/Functions/TestForecast.cs: Added a new test class TestForecast for the Forecast function. This class includes tests for basic functionality, handling of large numbers and arrays, error propagation, and a test replicating an example from the Microsoft support website.

[tonyqus]

Reference:
https://github.com/apache/poi/blob/trunk/poi/src/main/java/org/apache/poi/ss/formula/functions/Forecast.java
https://github.com/apache/poi/blob/trunk/poi/src/test/java/org/apache/poi/ss/formula/functions/TestForecast.java

[ken-swyfft]

I see that the ubuntu flavor of the tests are failing - but from what I can tell, that's happening on other PR's as well, so I'm assuming it's not something I broke in this PR?

[tonyqus]

I see that the ubuntu flavor of the tests are failing - but from what I can tell, that's happening on other PR's as well, so I'm assuming it's not something I broke in this PR?

Ubuntu test usually fails. No worries. It's not caused by your code.

[tonyqus]

LGTM