Reduce file operations on readin

[NGoetz]

Currently, we read twice through input files: once to identify event number and length, and once to actually create particles. The idea behind this was to allow for the filtering of event numbers, and to validate the filtering requested by the user (you can't say: I want event 24-56 if there are only 30 events). Additionally, this ensures even more that files are not corrupted. However, this is unnecessary overhead, especially for large event files, and should be replaced by single pass readin, at least as an option.