BUG: Fix bug with cache invalidation

[larsoner]

Before merging …

• Changelog has been updated (docs/source/changes.md)

Closes #748

First run of ds004229 with the default epochs_tmax=1:

[13:04:08] ┌╴🚀 preprocessing/_05_make_epochs Now running  👇
[13:04:08] │ ⏳️ preprocessing/_05_make_epochs sub-102 Loading filtered raw data from sub-102_task-amnoise_proc-filt_raw.fif
[13:04:09] │ ⏳️ preprocessing/_05_make_epochs sub-102 Creating task-related epochs …

Next run correctly shows as cached:

[13:09:25] ┌╴🚀 preprocessing/_05_make_epochs Now running  👇
[13:09:25] │ ✅ preprocessing/_05_make_epochs sub-102 Computation unnecessary (cached) …
[13:09:25] └╴🎉 preprocessing/_05_make_epochs Done running  👆 [1s]

Change tmax to 2 it correctly re-runs:

[13:17:08] ┌╴🚀 preprocessing/_05_make_epochs Now running  👇
[13:17:08] │ ⏳️ preprocessing/_05_make_epochs sub-102 Loading filtered raw data from sub-102_task-amnoise_proc-filt_raw.fif
[13:17:09] │ ⏳️ preprocessing/_05_make_epochs sub-102 Creating task-related epochs …

On this PR, changing back to 1 correctly re-runs:

[13:18:29] ┌╴🚀 preprocessing/_05_make_epochs Now running  👇
[13:18:29] │ 🔂 preprocessing/_05_make_epochs sub-102 Output file hash mismatch, will recompute …
[13:18:29] │ ⏳️ preprocessing/_05_make_epochs sub-102 Loading filtered raw data from sub-102_task-amnoise_proc-filt_raw.fif
[13:18:30] │ ⏳️ preprocessing/_05_make_epochs sub-102 Creating task-related epochs …

[agramfort]

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