Update DECIMAL_RE to allow scientific notation in auto inferred schemas

[pjmore]

Which issue does this PR close?

Closes #1215.

Rationale for this change

Allow the csv schema inference to handle scientific notation in floating point columns.

What changes are included in this PR?

Update DECIMAL_RE to match on numbers in the form 2e2, -2e-3, 3.4e-5.
Update some csv tests.

Are there any user-facing changes?

Columns that would have been inferred to be utf8 might infer to Float64

[matthewmturner]

@pjmore thank you for this. i tried out the regex and looks good.

[alamb]

Looks good to me -- thank you @pjmore

[codecov-commenter]

Codecov Report

Merging #1216 (aa5d946) into master (fcd37ee) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1216   +/-   ##
=======================================
  Coverage   82.70%   82.71%           
=======================================
  Files         175      175           
  Lines       51711    51712    +1     
=======================================
+ Hits        42769    42771    +2     
+ Misses       8942     8941    -1     

Impacted Files	Coverage Δ
arrow/src/csv/reader.rs	88.12% <100.00%> (+0.01%)	⬆️
parquet/src/encodings/encoding.rs	93.52% <0.00%> (-0.20%)	⬇️
arrow/src/array/transform/mod.rs	85.69% <0.00%> (+0.13%)	⬆️
arrow/src/datatypes/field.rs	54.10% <0.00%> (+0.30%)	⬆️

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[alamb]

@pjmore it appears there is a lint failure on this PR https://github.com/apache/arrow-rs/runs/4900531577?check_suite_focus=true

Can you please fix it?

[liukun4515]

@pjmore please fix lint.
LGTM

[alamb]

Thanks @pjmore and @liukun4515