Add basic dataset validator. Ref #165

[tompollard]

As discussed in #165, it would be helpful to have a pre-submission validation tool that helps contributors verify their datasets before uploading to PhysioNet.

This pull request adds a new validate module and command line interface. Users can now run physionet validate PATH to generate a report card for their dataset.

Validation Categories:

• Filesystem: File naming conventions, proprietary formats, hidden/temp files, version control artifacts
• Documentation: Required files (README.md by default)
• Integrity: CSV structure, encoding, duplicate columns, inconsistent rows
• Quality: Empty columns in CSV
• Privacy: Sensitive information (SSN, email, phone, ages >89), sensitive config files

An example report is below:

PhysioNet Dataset Validation Report
==================================================

Metadata:
  Dataset: organ-retrieval-and-collection-of-health-information-for-donation-orchid-2.1.1
  Validator version: 0.1.4
  Timestamp: 2025-12-12 20:58:28 UTC
  Total size: 682.6 MB (17 files)

Validation Results:
==================================================
✗ Filesystem (1 error, 3 warnings)
  ⚠ Data Description.csv - Filename contains spaces: Data Description.csv
  ⚠ OPOReferrals.mat - Proprietary file format detected: OPOReferrals.mat
  ⚠ DataDescription copy.dicom - Filename contains spaces: DataDescription copy.dicom
  ✗ CultureEvents****(.csv - Filename contains invalid characters ('*'): CultureEvents****(.csv
✗ Documentation (1 error)
  ✗ README.md - Required file not found: README.md
✓ Integrity
✓ Quality
✗ Privacy (1 error, 1 warning)
  ✗ credentials.json - Sensitive file detected: credential file
  ⚠ LICENSE.txt - Potential private information detected (email address)

Summary:
==================================================
3 errors, 4 warnings

✗ Dataset has errors that must be fixed before submission

Recommendations:
==================================================

Filesystem:
  ✗ Remove special characters from filename (use only letters, numbers, underscores, hyphens, and periods)
  ⚠ Replace spaces with underscores or hyphens (2 files)
  ⚠ MATLAB format; consider .csv, .zarr, .parquet, or .npy instead

Documentation:
  ✗ Add README.md to your dataset. At minimum, the file should include a title and a brief description of the package content.

Privacy:
  ✗ Remove 'credentials.json' from the dataset before submission
  ⚠ Review and remove or de-identify sensitive information

Note: A validation report (PHYSIONET_REPORT.md) has been saved in your
      dataset folder. Please include this file in your final submission.