feat: all essential RNTuple writing functionality

[ariostas]

This PR extend the writing functionality for RNTuples quite a bit. It adds the following:

• Lists of booleans
• Lists of strings
• Jagged arrays (including nested jagged arrays)
• Rectangular arrays
• Structs
• Lists of structs
• Optional types
• Union types

[ariostas]

All the functionality is now ready, I'll do a bit of cleanup tomorrow morning and it's ready to go.

[ariostas]

This is all I was planning to do for now. I ended up adding all the essential functionality that I could think of. There are still Awkward Form/Arrays that are not supported, but I think they are probably not too common and don't have direct translation to RNTuple fields, so I'll deal with them later. You can see from this test data that all essentials should be covered.

uproot5/tests/test_1395_rntuple_writing_lists_and_structs.py

Lines 16 to 43 in ce96300

	data = ak.Array(
	{
	"bool": [True, False, True],
	"int": [1, 2, 3],
	"float": [1.1, 2.2, 3.3],
	"jagged_list": [[1], [2, 3], [4, 5, 6]],
	"nested_list": [[[1], []], [[2], [3, 3]], [[4, 5, 6]]],
	"string": ["one", "two", "three"],
	"utf8_string": ["こんにちは", "⚛️💫🎆😀", "ǧ̸̛̫͍̰͖̟̈͛͑͆̆̌̃̉̅̄̔̈́̀̔͆̄͋̍͐͂̎͗̈́͒͘͝ͅö̴̮̝̪̬͎͚̜̖̜͖̞̤͕̙͂̀̀̊͛͑̈́͛͐͊͂͂̇͛̾̔͐͆͑͂̓̅̀͘͘͘̕͝͠͝͝ơ̶͍̙̻̾̈́̓̈́̀̅͑ḑ̷͚̠̹̗͉͙̞͇͕̼̲̥͉̯̞͕̲̻̞͗̓̃̊̅͗͊͊́̑̈́̎͋̇̓͛̅͜͜͠͝ͅb̷̢̢̨̨̛̛̘̠̞̰̺̘̰̖̺̞̱͇̰̙̲̱̪͕͎͉̖̞͇̹̮͙͋̀͑͂̈́̇͛̐͊̀̇͆̓̋̀̿̋̂̅̀̌̑̓̽͊̂͑̈̇̚͜͝y̶̗͇̠̞͚̦̮̦͈̹̥̋̓̓̈́̐̆̀̄̋̂̀̇͋̎̚͜͝ȩ̷̢̡͇̮̩̹̥̬̰͎͔̬̩̰̯͍̲͎̭͉̬̣̻̖͍̥̟̪͕̫̟̋̔̀͆̑̈́̐̃͐͌̍͒̔̈́̃̈́̐̔̾͊̿̓͆͑̚͜͝͝͝ͅ"],
	"regular": ak.Array(
	ak.contents.RegularArray(
	ak.contents.NumpyArray([1, 2, 3, 4, 5, 6, 7, 8, 9]), 3
	)
	),
	"numpy_regular": numpy.array([[1, 2, 3], [4, 5, 6], [7, 8, 9]]),
	"struct": [{"x": 1, "y": 2}, {"x": 3, "y": 4}, {"x": 5, "y": 6}],
	"struct_list": [
	[{"x": 1}, {"x": 2}],
	[{"x": 3}, {"x": 4}],
	[{"x": 5}, {"x": 6}],
	],
	"tuple": [(1, 2), (3, 4), (5, 6)],
	"tuple_list": [[(1,), (2,)], [(3,), (4,)], [(5,), (6,)]],
	"optional": [1, None, 2],
	"union": [1, 2, "three"],
	"optional_union": [1, None, "three"],
	}
	)

It's definitely not perfect, but I'll improve things later. I think it would be good to advertise that we now have experimental writing functionality and see what feedback we start getting.

[ianna]

@ariostas - Great! It looks good to me. Please, merge if you are done with it.

[ianna]

wow!

[ianna]

I guess, this will fail after your PR is merged and released?

[ariostas]

Yeah, let me fix it now

[ariostas]

Actually, this still works after the other PR since there are some other parts in the RNTuple reading part that need to be updated. I'll work on all those things in a refactoring of the reading part that I'm planning for the next couple of weeks.