era export: make a CLI tool for the accumulator calculator

[filoozom]

Is your feature request related to a problem? Please describe.

Kind of related to #8454: it would be great to have a version of the accumulator calculator as a CLI tool. This would help for verification of the files and to generate missing accumulators.txt files if necessary. For example, when an import of a big chunk of era files fails for any reason, the file isn't generated, and the whole process has to be started from scratch.

Describe the solution you'd like

Have a CLI tool to calculate accumulators.txt files for a given directory or era1 file.

Describe alternatives you've considered

None really, except for re-exporting the whole thing in case of failures or the hack described in #8454 to keep the accumulators.txt file up to date.