Add support for savestreaming method, take 2

[davidanthoff]

Replaces #53.

The documentation for the API in FileIO.jl is really incomplete, but I think this PR is how it should be done (I tracked down some packages from the person who initially added this feature to FileIO.jl and looked how they are doing it).

The API from a user point of view is:

s = savestreaming(fname, df1)
# OR
s = savestreaming(fname)
# If one doesn't want to write any data at this point.

write(s, df2)
write(s, df2)

close(s)

@lrennels, can you adapt your code to this new API, test whether things work, and then I'll merge this PR here?

[lrennels]

Sure I'll do that this weekend thank you!

[lrennels]

@davidanthoff I use a dictionary to keep track of the streams, that was previously initialized as

streams::Dict{String, FileIO.Stream}

this now causes a type error, is the following correct now? I could probably also just not type parameterize it.

streams::Dict{String, CSVFiles.CSVFileSaveStream{IOStream}}

[codecov-io]

Codecov Report

Merging #59 into master will decrease coverage by 3.26%.
The diff coverage is 72.72%.

@@            Coverage Diff             @@
##           master      #59      +/-   ##
==========================================
- Coverage     100%   96.73%   -3.27%     
==========================================
  Files           2        2              
  Lines          81       92      +11     
==========================================
+ Hits           81       89       +8     
- Misses          0        3       +3

Impacted Files	Coverage Δ
src/csv_writer.jl	94.23% <72.72%> (-5.77%)	⬇️

---

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 03dc201...76afde8. Read the comment docs.

[lrennels]

@davidanthoff ok all good you can merge and tag!