ENH: Allow save-time passing of on-disk dtype to save functions

[effigies]

A common idiom is to use a higher-precision internal data type before saving to a lower-precision on-disk data type. The set_data_dtype() method is a little clunky. This permits the following:

nii.to_filename(fname, dtype="int16")
nii.to_bytes(dtype="int16")
nb.save(nii, fname, dtype="int16")

Only implemented for Analyze-like images here. Related to discussion in #1046, but does not (currently) warn for int64 data being saved without a dtype argument.

[codecov]

Codecov Report

Merging #1083 (325b658) into master (2937a3a) will decrease coverage by 0.00%.
The diff coverage is 85.71%.

@@            Coverage Diff             @@
##           master    #1083      +/-   ##
==========================================
- Coverage   92.32%   92.31%   -0.01%     
==========================================
  Files          99       99              
  Lines       12205    12221      +16     
  Branches     2388     2392       +4     
==========================================
+ Hits        11268    11282      +14     
- Misses        613      614       +1     
- Partials      324      325       +1     

Impacted Files	Coverage Δ
nibabel/analyze.py	97.95% <66.66%> (-0.57%)	⬇️
nibabel/filebasedimages.py	89.36% <100.00%> (ø)
nibabel/loadsave.py	93.84% <100.00%> (ø)
nibabel/affines.py	100.00% <0.00%> (ø)
nibabel/streamlines/tractogram.py	99.68% <0.00%> (+<0.01%)	⬆️
nibabel/streamlines/array_sequence.py	97.97% <0.00%> (+0.02%)	⬆️

---

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 2937a3a...325b658. Read the comment docs.

[effigies]

Combining into #1082.