Fix shift in time between fields and beam in IO

[SeverinDiederichs]

This PR is the first step to fix the physical time mentioned in #404.

Previously, the physical time of the beam and the fields was shifted:
For max_step = 1, previously, the beam at t = 0 was neglected and the fields at t=0 were written to iteration 1, as well as the beam at t = 1

With this PR, the beam and the fields are correct: For max_step = 1, both fields and beams are written to iteration 0 at t = 0 and iteration 1 at t = t1.

However, this comes with an important difference: max_step = 1, now means 2 plasma swipes, one for t = 0 and one for t = t1.

Therefore, the max_step was adjusted in the CI tests.

• Small enough (< few 100s of lines), otherwise it should probably be split into smaller PRs
• Tested (describe the tests in the PR description)
• Runs on GPU (basic: the code compiles and run well with the new module)
• Contains an automated test (checksum and/or comparison with theory)
• Documented: all elements (classes and their members, functions, namespaces, etc.) are documented
• Constified (All that can be const is const)
• Code is clean (no unwanted comments, )
• Style and code conventions are respected at the bottom of https://github.com/Hi-PACE/hipace
• Proper label and GitHub project, if applicable