Add `options_inpgen` input to SCF workchain

[janssenhenning]

Allows to give differing options for the inpgen code from the fleur executable.

Fixes #210

[codecov[bot]]

Codecov Report

Patch coverage: 83.33% and project coverage change: -0.01 :warning:

Comparison is base (21f7a10) 77.56% compared to head (d74396a) 77.55%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop #211 +/- ## =========================================== - Coverage 77.56% 77.55% -0.01% =========================================== Files 122 122 Lines 15430 15429 -1 =========================================== - Hits 11968 11966 -2 - Misses 3462 3463 +1 ``` | [Impacted Files](https://app.codecov.io/gh/JuDFTteam/aiida-fleur/pull/211?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuDFTteam) | Coverage Δ | | |---|---|---| | [aiida\_fleur/workflows/scf.py](https://app.codecov.io/gh/JuDFTteam/aiida-fleur/pull/211?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuDFTteam#diff-YWlpZGFfZmxldXIvd29ya2Zsb3dzL3NjZi5weQ==) | `74.73% <80.00%> (-0.23%)` | :arrow_down: | | [tests/tools/test\_StructureData\_util.py](https://app.codecov.io/gh/JuDFTteam/aiida-fleur/pull/211?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuDFTteam#diff-dGVzdHMvdG9vbHMvdGVzdF9TdHJ1Y3R1cmVEYXRhX3V0aWwucHk=) | `99.12% <100.00%> (ø)` | |

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