replace lambda function for mac compatibility

[rvhonorato]

when doing haddocking/haddock3#1480 I saw the actions were failing on macos with the error:

_pickle.PicklingError: Can't pickle <function HDF5DataSet.<lambda> at 0x1130d62a0>: attribute lookup HDF5DataSet.<lambda> on deeprank_gnn.DataSet failed

The solution to this is very simple, this HDF5DataSet is taking a lambda function as an argument into an object that is being passed to a multiprocessing worker. This works fine or linux but mac (for some reason) cannot handle the lambda assignment directly - so the fix is to simply pass it as a function instead.

[codacy-production]

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	Coverable lines	Covered lines	Coverage
Common ancestor commit (8305a74)	Report Missing	Report Missing	Report Missing
Head commit (2919c47)	254	228	89.76%

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Pull request (#32)	0	0	∅ (not applicable)

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[ntxxt]

Looks good to me!

I didn't know lambda function is the cause of this.