Fixes remaining bugs in supplychain module

[spjuhel]

This PR fixes #187

It:

• adds computation of required attributes of the MRIOT when creating them.
• Fixes the index error that happened when merging results after computation.

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[spjuhel]

This should be very very quick to review (5 min tops), I changed only a few lines.

[ValentinGebhart]

The changes look reasonable to me. Sorry for the late review!

[emanuel-schmid]

The fixes look really neat. (:
However - strictly speaking we ought to have tests for them.

[spjuhel]

Done!
I actually improved the flow for the mriot member checking.

For the index name test, the mocked dataframe values used in test_calc_indirect_impacts were "too informative" with respect to the value actually returned by their non-mocked corresponding methods.