Add default values to data writer

[slacrherbst]

This adds the ability to provide default values for bufferSize and maxFileSize to the DataWriter class.

This also removes the fields from the pydm widget as the previous method to conditionally include the fields was not working.

[codecov-commenter]

Codecov Report

Merging #947 (acb426e) into pre-release (f8174c3) will increase coverage by 0.46%.
The diff coverage is 100.00%.

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@@               Coverage Diff               @@
##           pre-release     #947      +/-   ##
===============================================
+ Coverage        25.54%   26.01%   +0.46%     
===============================================
  Files               72       72              
  Lines             7849     7835      -14     
  Branches          1173     1169       -4     
===============================================
+ Hits              2005     2038      +33     
+ Misses            5683     5633      -50     
- Partials           161      164       +3     

Impacted Files	Coverage Δ
python/pyrogue/pydm/widgets/data_writer.py	0.00% <ø> (ø)
python/pyrogue/_DataWriter.py	35.13% <100.00%> (ø)

... and 2 files with indirect coverage changes