Change EventHeader check to be non fatal

[simonge]

Briefly, what does this PR introduce?

Allows running of EICrecon on files without an EventHeader collection. Files with a single/small subset of output collections can have analysis continued without needing to manually persist EventHeader.

A warning has been added.

What kind of change does this PR introduce?

• Bug fix (issue #__)
• New feature (issue Make EventHeader collection non-essential #1696)
• Documentation update
• Other: __

Please check if this PR fulfills the following:

• Tests for the changes have been added
• Documentation has been added / updated
• Changes have been communicated to collaborators

Does this PR introduce breaking changes? What changes might users need to make to their code?

No

Does this PR change default behavior?

No

[github-actions]

Capybara summary for PR 1698

• rec_dis_10x100_minQ2=0_craterlake
• rec_dis_10x100_minQ2=1000_craterlake_tracking_only
• rec_dis_18x275_minQ2=0_craterlake_18x275
• rec_dis_18x275_minQ2=1000_craterlake_18x275
• rec_dis_5x41_minQ2=0_craterlake_5x41
• rec_e_1GeV_20GeV_craterlake
• rec_pi_1GeV_20GeV_craterlake
  ^{_{Last updated 2025-01-08T20:07-05:00 0151a46}}

[veprbl]

This is probably okay for now, but only because we don't rely on those numbers. Once conditions DBs are implemented, this will have to be provided somehow.