Set initial epoch number when resuming

Currently the epoch starts from 1 when resuming. It is better to be able to set arbitrary number by argument.
(Or automatically taken from the file name.)

Start by locating the resume/checkpoint handling and where the epoch counter is initialized. Review how command-line arguments and checkpoint filenames are currently parsed before deciding how an arbitrary starting epoch should be supplied. Done means resuming can start from the requested epoch, with behavior covered for the supported input path.

Written by the indexing model from the issue text.