fix: Update MemoryReader to read more than one field

[HCookie]

Description

MemoryReader could only read one field,
Use totalLength to update index and keep reading.

Contributor Declaration

By opening this pull request, I affirm the following:

• All authors agree to the Contributor License Agreement.
• The code follows the project's coding standards.
• I have performed self-review and added comments where needed.
• I have added or updated tests to verify that my changes are effective and functional.
• I have run all existing tests and confirmed they pass.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 85.71429% with 5 lines in your changes missing coverage. Please review.

Please upload report for BASE (develop@a488583). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
eccodes/highlevel/reader.py	0.00%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             develop     #129   +/-   ##
==========================================
  Coverage           ?   33.89%           
==========================================
  Files              ?       16           
  Lines              ?     2679           
  Branches           ?      197           
==========================================
  Hits               ?      908           
  Misses             ?     1755           
  Partials           ?       16           

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