pynbody/genetIC: Version 1.3

New features Introduce 'gene splicing', i.e. the ability to cut together different realisations of the overdensity field. An accompanying paper will be posted to arXiv shortly (Cadiou, Pontzen, Peiris & Lucie-Smith 2021). Add ability to output multiple gadget files (essential for large ICs due to limitations of the gadget format) Bug fixes Addresses issue #90, where placement of zoom windows near the edge of the box could result in errors Addresses issue #85, where Zel'dovich displacement calculations could cause a bounds overrun erro