Changed the accucor and isocorr validation file fields into a single field

[hepcat72]

Summary Change Description

• Improved the form's clean method so that errors would be associated with their fields.
• Added a check to the form's clean method to ensure that the files are excel and that the peak annotation files were specifically either accucor or isocorr (individually).
• Changed the order of the set_files args and made the sample arg optional.
• Added methods: is_accucor and is_isocorr to the AccucorDataLoader class.
• Changed the way things were imported from django.forms so that it would look cleaner.

Affected Issues/Pull Requests

• Partially addresses #829
• Merges into PR #924
• Next PR: #926

Review Notes

See comments in-line.

Checklist

This pull request will be merged once the following requirements are met. The author and/or reviewers should uncheck any unmet requirements:

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  • Minimum approvals: 1
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  • All blocking issues resolved by reviewers
  • Specific reviewers: @add_username_here
  • Review period: 2 days
• Associated issue/pull request requirements:
  • [x] All requirements in affected issues marked "resolved" are satisfied
  • [x] All required pull requests are merged (or none)
• Basic requirements
  • [x] All linters pass
  • [x] All tests pass
  • [x] All conflicts resolved
• Overhead requirements
  • [x] New/changed method tests implemented/updated (or no change)
  • [ ] Updated Unreleased section of changelog.md (or no change)
  • [x] Migrations created & committed (or no change)