Adds ProjDataInMemory::read_from_file to return a ProjDataInMemory object

[robbietuk]

Changes in this pull request

Previously,

auto pd1 = ProjData::read_from_file(fname);
auto pd2 = ProjDataInMemory::read_from_file(fname);

would both return a ProjData object.

This change, overwrites the ProjData::read_from_file() method for ProjDataInMemory. This allows ProjDataInMemory::read_from_file() to return a ProjDataInMemory object.

Testing performed

CI

Related issues

Checklist before requesting a review

• [] I have performed a self-review of my code
• [] I have added docstrings/doxygen in line with the guidance in the developer guide
• [] I have implemented unit tests that cover any new or modified functionality (if applicable)
• [] The code builds and runs on my machine
• [] documentation/release_XXX.md has been updated with any functionality change (if applicable)

[robbietuk]

@KrisThielemans Are you happy with this implementation? It can be squash merged.

[KrisThielemans]

actually, the open_mode should be removed (or at least ignored). ProjDataInMemory will always be read/write, and we're making a copy anyway. The initial ProjData::read_from_file should always be read-only (more chance that it works), which is the default.

Also, add something to release_6.3.htm