This function (available only in Processing expressions for now),
allows loading a map layer via a source string and provider name.
It is designed to allow use of the expression functions which
directly reference map layers (such as the aggregate functions)
with a hardcoded layer path, eg. then permitting these functions
to be used outside of a project (such as via the qgis_process tool)
* mesh expression $vertex_as_point $vertex_Z_value
* functions help
* SIP, doc and indentation
* fix SIP
* fix help file
* change function name and add Meshes group description
* fix typo
* fix strings and docs
* typo
For the c++ api dox this expands to "\c nullptr" (the
\c directive indicates a code literal value), and for sipify/Python
it expands to ``None`` (`` is sphinx annotation for literal values)
Makes for nicer dox for both c++ and Python!
