Clarify documentation of filer input variable

[martinholmer]

The name of the input variable, filer, is misleading because some might think a variable with that name is an output variable that indicates whether or not the unit filed an income tax return in the Tax-Calculator simulation. This would appear to be what happened in issue #2099, which was raised by one of our most advanced users.

Do people think we should change this variable name in the taxdata and Tax-Calculator repositories? If so, what should the new variable name be? Something like data_source? Other ideas?

Can people suggest better records_variables.json documentation text than what is proposed in this pull request?

The focus of this pull request has changed as described here and here.

@MattHJensen @andersonfrailey @hdoupe @evtedeschi3

[codecov-io]

Codecov Report

Merging #2102 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2102   +/-   ##
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  Coverage   99.91%   99.91%           
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  Files          16       16           
  Lines        3523     3523           
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  Hits         3520     3520           
  Misses          3        3

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[martinholmer]

Tax-Calculator pull request #2102 has been changed to eliminate the filer variable from the Records.USABLE_READ_VARS set. This is an interim state of affairs lasting only until taxdata issue 288 is resolved, in which case the renamed and redefined variable will be reintroduced into the Records.USABLE_READ_VARS set.