Handle metadata for trailing empty fields

[davidorme]

The sdv.field.Dataworksheet class keeps an internal representation of the whole worksheet read from file, which can include trailing blank fields. Those are kept to validate that the rows below are empty but the returned metadata and the reported worksheet size do not remove the empty fields. This PR:

• internally records the number of trailing empty fields to allow accurate reporting,
• updates the to_dict method conversion of the list of field objects to remove trailing empty fields from the dataworksheet metadata.

This PR fixes #60

[codecov-commenter]

Codecov Report

Merging #61 (1870f90) into release/3.0.0 (6cb2449) will increase coverage by 0.00%. The diff coverage is 75.00%.

@@              Coverage Diff               @@
##           release/3.0.0      #61   +/-   ##
==============================================
  Coverage          67.64%   67.65%           
==============================================
  Files                 12       12           
  Lines               3576     3580    +4     
==============================================
+ Hits                2419     2422    +3     
- Misses              1157     1158    +1     

Impacted Files	Coverage Δ
safedata_validator/field.py	93.48% <75.00%> (-0.10%)	:arrow_down:

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