field filter array dereference

[Liyuan-Chen-1024]

fix #227

[Liyuan-Chen-1024]

[] how to separate two cases: apply_filter, and apply_index?
Should we use logic below: if filter_to_apply is boolean type, then [] will call apply_filter, otherwise, it will call apply_index?

[atbenmurray]

You can forward to the self.data getitem in the general case and then wrap the result in a mem field

[ericspod]

The data can be acquired with self.data[slice] I think, this can then be put into a mem field.

[ericspod]

You can get an array of a single value with self.data[[item]] and then put that into a mem field.

[atbenmurray]

same for this

[codecov-commenter]

Codecov Report

Merging #302 (6a6f15f) into master (bc66b84) will increase coverage by 0.04%.
The diff coverage is 96.15%.

@@            Coverage Diff             @@
##           master     #302      +/-   ##
==========================================
+ Coverage   83.39%   83.43%   +0.04%     
==========================================
  Files          22       22              
  Lines        6191     6213      +22     
  Branches     1247     1252       +5     
==========================================
+ Hits         5163     5184      +21     
  Misses        733      733              
- Partials      295      296       +1     

Impacted Files	Coverage Δ
exetera/core/fields.py	91.09% <95.83%> (+0.06%)	⬆️
exetera/core/dataframe.py	87.36% <100.00%> (ø)

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