Added shallow search for data.table in tables()

[manmita]

Closes #2606

added arg shallow_search = TRUE to tables() one for shallow search
if shallow_search is false then its the previous code
else we loop through list-like objects using is.list and which are not data.table or data.frame

added name for the nested list found parent[[1]] or parent$child
pre-allocating info to avoid reallocation cost

[manmita]

Hello,

I created a new PR in replacement of #7568

Reasons: There was some git issue there and the merge became too complex and I changed the algo because I didnt know previously that rbind or cbind would cost for re-allocation

The current PR considers that part and avoids appends

Previous PR : creating seperate data.table called info and rbind at the end
This PR: pre-allocates for a total-sized data.table and fills the info

[manmita]

In reply to previous comment of @jangorecki

An example of when this new feature could be useful?

To support lists which occur due to split.data.table or fread like the following

list(data.table(a = 1, b = 4:6)),
      data.table(a = 2, b = 7:10))

The original code supported data.table() top level and this code adds support for list(data.table) if the arg shallow_search = TRUE

[manmita]

Example of the original code and the new feature is as follows

> A = list(data.table(a = 1, b = 4:6),
      data.table(a = 2, b = 7:10))
> B = list(data.table(a = 1, b = 4:6), 1:5)
> C = data.table(a = 1, b = 4:6)
> tables()
   NAME NROW NCOL MB COLS    KEY
1:    C    3    2  0  a,b [NULL]
Total: 0MB using type_size
> tables(shallow_search = TRUE)
     NAME NROW NCOL MB COLS    KEY
1: A[[1]]    3    2  0  a,b [NULL]
2: A[[2]]    4    2  0  a,b [NULL]
3: B[[1]]    3    2  0  a,b [NULL]
4:      C    3    2  0  a,b [NULL]
Total: 0MB using type_size
> D = list(d = data.table(a = 1, b = 4:6), x = 1:5)
> tables(shallow_search = TRUE)
     NAME NROW NCOL MB COLS    KEY
1: A[[1]]    3    2  0  a,b [NULL]
2: A[[2]]    4    2  0  a,b [NULL]
3: B[[1]]    3    2  0  a,b [NULL]
4:      C    3    2  0  a,b [NULL]
5:    D$d    3    2  0  a,b [NULL]
Total: 0MB using type_size

tables() work same as before and tables(shallow_search = TRUE) searches 1 level

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.02%. Comparing base (f3b166b) to head (1952290).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #7580   +/-   ##
=======================================
  Coverage   99.02%   99.02%           
=======================================
  Files          87       87           
  Lines       16754    16793   +39     
=======================================
+ Hits        16591    16630   +39     
  Misses        163      163           

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[github-actions]

No obvious timing issues in HEAD=feat/adding_list_search_to_tables

Generated via commit 1952290

Download link for the artifact containing the test results: ↓ atime-results.zip

Task	Duration
R setup and installing dependencies	2 minutes and 56 seconds
Installing different package versions	22 seconds
Running and plotting the test cases	4 minutes and 3 seconds