Hash change guards

[tclose]

Types of changes

• New feature (non-breaking change which adds functionality)

Summary

Addresses #681 issue

• Detects changes in the hashes of task/workflow inputs and raises informative error messages identifying which fields have changed
• Reinstates behaviour of "blocked tasks" unittest, so it once again triggers "graph is not empty" exception
• On "graph not empty" exception, check Workflow.inputs._graph_checksums to determine which task's checksums have changed and raise informative message

Checklist

• [x] I have added tests to cover my changes (if necessary)

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 97.05882% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 83.93%. Comparing base (0e66136) to head (ff281aa).

Files	Patch %	Lines
pydra/engine/submitter.py	88.23%	2 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #698 +/- ## ========================================== + Coverage 83.65% 83.93% +0.27% ========================================== Files 24 24 Lines 4986 5029 +43 Branches 1416 1429 +13 ========================================== + Hits 4171 4221 +50 + Misses 809 802 -7 Partials 6 6 ``` | [Flag](https://app.codecov.io/gh/nipype/pydra/pull/698/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=nipype) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/nipype/pydra/pull/698/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=nipype) | `83.93% <97.05%> (+0.27%)` | :arrow_up: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=nipype#carryforward-flags-in-the-pull-request-comment) to find out more.

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[tclose]

I believe this is good for review now. The new SLURM test keeps on getting stuck somewhere but I assume that it isn't related

[tclose]

NB: I'm not sure why the coverage is down. It seems to be an issue with the dask submitter code

[djarecka]

indeed looks like the coverage has dropped, what is weird, since for some time we didn't test Dask at all...

[tclose]

indeed looks like the coverage has dropped, what is weird, since for some time we didn't test Dask at all...

I don't trust codecov in this instance (and others). The dask test is passing so it couldn't be missing the Dask submitter, which is where the bulk of the coverage drop comes from (otherwise it would be a coverage increase I believe)

[tclose]

@djarecka I believe this is ready to merge now

[djarecka]

I'm good with merging, thank you @tclose !