44 readme code fails

[radlfabs]

This fixes issues with code in the readme.
Tests are added for the readme.
Thanks to Siegbert for pointing me in the right direction.

Also fixes problems where tests in the docs did not xfail with the correct neptune exception.

[github-actions]

Coverage Report

File	Stmts	Miss	Cover	Missing
flexcv
core.py	190	5	97%	280–281, 320–321, 566
fold_logging.py	56	2	96%	136–139
interface.py	329	42	87%	125, 128, 182, 184, 187, 190–191, 195, 320, 325, 328, 358, 422, 427, 605, 608, 618, 622, 629, 643, 646, 664, 713, 796–797, 800–801, 877, 886, 891, 896, 900–930
merf.py	154	18	88%	169, 172, 191, 235, 246–248, 306, 384, 400–413, 417–422, 428
metrics.py	12	1	92%	72
model_mapping.py	23	3	87%	141–143
model_postprocessing.py	219	16	93%	123, 170, 201, 227, 266, 288–293, 303–305, 405, 552, 647
model_selection.py	70	2	97%	61, 65
models.py	95	4	96%	197, 209, 219, 223
plot.py	126	21	83%	101–102, 111–116, 175–181, 194–200, 236
repeated.py	115	15	87%	30–32, 217–223, 258–260, 272, 285–288, 360
results_handling.py	75	7	91%	79, 254–255, 260, 264, 267, 271
run.py	30	1	97%	50
split.py	58	5	91%	103–106, 131–134, 154
utilities.py	82	26	68%	24, 157–163, 174–210
yaml_parser.py	53	3	94%	118, 205–220
TOTAL	1867	171	91%

Tests	Skipped	Failures	Errors	Time
476	0 💤	0 ❌	0 🔥	5m 0s ⏱️