Fix ParticleHistogram2D Value Function

[Felice27]

The documentation and code comments list ParticleHistogram2D's value_function as an optional flag, but when omitted, the code still calls the parser and accumulates 0 in every bin, yielding a histogram of all 0's without any warning or error message. This is inconsistent with the 1D histogram, which defaults to summing over particle weight w.

Changes implemented:

• m_do_parser_value is assigned to `false' in the header if not declared
• In the constructor, value_function defaults to "w" when not provided and always builds a parser from the resolved string
• value_function is written to the OpenPMD file metadata
• Docs: clarify that the 2D parsers receive 8 arguments (t, x, y, z, ux, uy, uz, w) instead of 7; update wording so that "value" and "filter" definitions aren't conflated.

This should patch an issue I was encountering where 2D Particle Histogram writing produced all-0 arrays if value function was not manually set to w.

Edited: found an old issue of a user encountering an issue with the default implementation. Fixes #4540