[MRG] construct mgh image only when vox2ras_tkr is needed

[jasmainak]

PR Description

closes #238

cc @alexrockhill

Merge checklist

Maintainer, please confirm the following before merging:

• All comments resolved
• This is not your own PR
• All CIs are happy
• PR title starts with [MRG]
• whats_new.rst is updated (not updated because write_anat was fixed in the same release cycle)
• PR description includes phrase "closes <#issue-number>"
• Commit history does not contain any merge commits

[sappelhoff]

Thanks @jasmainak this looks cool. I particularly like how many lines can be saved with the new _validate_bids fixture!

Could it be that the tests fail due to the use of _TempDir? See my similar comment here: #231 (review)

[jasmainak]

The error seems related to a new commit in the validator: bids-standard/bids-validator@4e6a5b3. Because the tests passed for the older version that I had on my computer. Unfortunately I can't seem to install the latest version for some reason.

[codecov-io]

Codecov Report

Merging #241 into master will decrease coverage by <.01%.
The diff coverage is 89.47%.

@@            Coverage Diff             @@
##           master     #241      +/-   ##
==========================================
- Coverage   98.26%   98.25%   -0.01%     
==========================================
  Files          15       15              
  Lines        1552     1545       -7     
==========================================
- Hits         1525     1518       -7     
  Misses         27       27

Impacted Files	Coverage Δ
mne_bids/write.py	97.06% <100%> (+0.01%)	⬆️
mne_bids/tests/test_write.py	98.79% <87.87%> (-0.04%)	⬇️

---

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[codecov-io]

Codecov Report

Merging #241 into master will decrease coverage by <.01%.
The diff coverage is 89.47%.

@@            Coverage Diff             @@
##           master     #241      +/-   ##
==========================================
- Coverage   98.26%   98.25%   -0.01%     
==========================================
  Files          15       15              
  Lines        1552     1545       -7     
==========================================
- Hits         1525     1518       -7     
  Misses         27       27

Impacted Files	Coverage Δ
mne_bids/write.py	97.06% <100%> (+0.01%)	⬆️
mne_bids/tests/test_write.py	98.79% <87.87%> (-0.04%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 4a5fccb...a14c6a6. Read the comment docs.

[jasmainak]

okay comments addressed @sappelhoff . I added a link. There is some weirdness in the way nibabel treats these header objects -- even if t1w.header['xyzt_units'] == 0 it's not written to disk. I would have imagined it should have the key missing or something. Not sure if it's worth digging more into this. @alexrockhill is our beta tester -- so if he hits another problem, we will fix it as it comes.

[sappelhoff]

Cool, thanks @jasmainak !

[mghneurotherapeutics]

The dataset I was testing with did not throw any errors or warnings when I tested it with the updates. Looks great, thanks Mainak!