Re-add explicit instructions to install dcm2niix "manually" and remove it from install_requires

[yarikoptic]

Closes #648

[codecov[bot]]

Codecov Report

Patch and project coverage have no change.

Comparison is base (71f4dbe) 81.36% compared to head (b530ddc) 81.36%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #651 +/- ## ======================================= Coverage 81.36% 81.36% ======================================= Files 42 42 Lines 3837 3837 ======================================= Hits 3122 3122 Misses 715 715 ``` | [Impacted Files](https://codecov.io/gh/nipy/heudiconv/pull/651?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=nipy) | Coverage Δ | | |---|---|---| | [heudiconv/dicoms.py](https://codecov.io/gh/nipy/heudiconv/pull/651?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=nipy#diff-aGV1ZGljb252L2RpY29tcy5weQ==) | `85.77% <ø> (ø)` | | | [heudiconv/info.py](https://codecov.io/gh/nipy/heudiconv/pull/651?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=nipy#diff-aGV1ZGljb252L2luZm8ucHk=) | `100.00% <ø> (ø)` | | Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=nipy). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=nipy)

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[github-actions[bot]]

:rocket: PR was released in v0.12.1 :rocket:

[yarikoptic]

Doh, will need to follow up with another PR/release size forgot to regenerate dockerfile