Support reading streaming from online .mtx or .mtx.gz files

[yuehhua]

Resolved #42

[codecov-commenter]

Codecov Report

Merging #54 (b205208) into master (bf9cc4f) will increase coverage by 3.99%.
The diff coverage is 95.86%.

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@@            Coverage Diff             @@
##           master      #54      +/-   ##
==========================================
+ Coverage   92.20%   96.20%   +3.99%     
==========================================
  Files           4        5       +1     
  Lines         154      237      +83     
==========================================
+ Hits          142      228      +86     
+ Misses         12        9       -3     

Impacted Files	Coverage Δ
src/MatrixMarket.jl	100.00% <ø> (ø)
src/format.jl	91.52% <91.52%> (ø)
src/mmread.jl	100.00% <100.00%> (+6.41%)	⬆️
src/mmwrite.jl	100.00% <100.00%> (+2.27%)	⬆️
src/mminfo.jl	87.50% <0.00%> (+6.85%)	⬆️

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[tkonolige]

Could you explain what you are trying to do with this PR. I'm not sure how I understand how it is related to reading from streams. It seems like having mmread(stream :: IO) already solves that problem.

[yuehhua]

Sorry for not explain this clearly. To support streaming, we not only have to support the interface like mmread(stream :: IO) , but also the core behavior of streaming, which is data is not transmitted by chunk, instead, data is transmitted by bytes. We couldn't expect we could fetch whole trunk of data, since data stream could be paused by internet. Thus, we should be able to support more fine-grained unit than a .mtx file we expected before. A single line in a .mtx file is supported in this PR in case that internet is gone and we could still get partial data in MMReader. The same reason to MMwriter. To save data for different format, CoordinateFormat and ArrayFormat are supported for sparse matrices and dense matrices, respectively. These two formats save single line of data into the format and could be fetched after internet is paused. They also support Iterable to iterate over data.

[tkonolige]

Doesn't the IO interface support streaming? The use case here seems to be resuming downloads but that seems like a really niche use case. Do you have an example of how this is used (like with a library that resumes downloads)? Overall I think added complexity to the code is not worth it.

[yuehhua]

Would I put the example to README? Or other place?

I think the code complexity would stop at this PR. It is sufficient complex.