fix: correct misleading error message in MidpointIntegration

[shaked-shlomo]

Description

In Maths/MidpointIntegration.js the guard rejects non-positive N:

if (N <= 0) {
  throw Error('N has to be >= 2')
} // check if N > 0

The thrown message says 'N has to be >= 2', but:

• the check only rejects N <= 0,
• the inline comment says check if N > 0, and
• the function docstring states "N must be > 0".

N = 1 is a valid number of subintervals for the midpoint rule, so the >= 2 message is incorrect and misleading. This changes it to 'N has to be > 0' to match the actual validation and the documentation.

(Note: the analogous >= 2 message in SimpsonIntegration.js is correct there, since Simpson's rule additionally requires an even N, so the minimum is 2 — left unchanged.)

Checklist

• Fix is a single, self-contained change.
• No existing test asserts the old message.
• No behavior change beyond the error string.

🤖 Generated with Claude Code

[codecov-commenter]

Codecov Report

❌ Patch coverage is 0% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 85.91%. Comparing base (5c39e87) to head (2e1ba82).

Files with missing lines	Patch %	Lines
Maths/MidpointIntegration.js	0.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1902   +/-   ##
=======================================
  Coverage   85.91%   85.91%           
=======================================
  Files         379      379           
  Lines       19778    19778           
  Branches     3016     3016           
=======================================
  Hits        16993    16993           
  Misses       2785     2785           

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