By Alessandra Prentice and Sergiy Karazy DYMER, Ukraine (Reuters) – Volodymyr Khropun and Yulia Ivannikova-Katsemon say they were helping people flee villages on the front line in northern Ukraine when they were detained by Russian soldiers over two days in March. Both said they were then held with around 40 other captives on the concrete floor of a nearby factory, their hands bound. Nearly a week later they were transferred in a military truck to Belarus, and on to detention centres in Russia, they said. Khropun, an electrical engineer, and Ivannikova-Katsemon, an emergency services dispatche…