Disallow constructing frames from a ColumnAccessor

[shwina]

Constructing a DataFrame from a ColumnAccessor previously had unintended side-effects:

In [1]: import cudf

In [2]: a = cudf.DataFrame({'a': [1, 2, 3]})

In [3]: a._data['a'].__cuda_array_interface__
Out[3]:
{'shape': (3,),
 'strides': (8,),
 'typestr': '<i8',
 'data': (140409137266688, False),
 'version': 1}

In [4]: a[['a']]
Out[4]:
   a
0  1
1  2
2  3

In [5]: a._data['a'].__cuda_array_interface__
Out[5]:
{'shape': (3,),
 'strides': (8,),
 'typestr': '<i8',
 'data': (140409137267200, False),
 'version': 1}

In a discussion with @galipremsagar - we decided that it's probably best not to handle ColumnAccessor in the frame constructors.

• Remove special handling of ColumnAccessor in Frame constructors
• Collapse Series.copy() and DataFrame.copy() into a single Frame.copy()

[codecov]

Codecov Report

Merging #7298 (02dcd24) into branch-0.18 (8860baf) will increase coverage by 0.12%.
The diff coverage is n/a.

@@               Coverage Diff               @@
##           branch-0.18    #7298      +/-   ##
===============================================
+ Coverage        82.09%   82.22%   +0.12%     
===============================================
  Files               97      100       +3     
  Lines            16474    16964     +490     
===============================================
+ Hits             13524    13948     +424     
- Misses            2950     3016      +66     

Impacted Files	Coverage Δ
python/cudf/cudf/__init__.py	100.00% <ø> (ø)
python/cudf/cudf/_fuzz_testing/avro.py	0.00% <ø> (ø)
python/cudf/cudf/_fuzz_testing/csv.py	0.00% <ø> (ø)
python/cudf/cudf/_fuzz_testing/fuzzer.py	0.00% <ø> (ø)
python/cudf/cudf/_fuzz_testing/io.py	0.00% <ø> (ø)
python/cudf/cudf/_fuzz_testing/json.py	0.00% <ø> (ø)
python/cudf/cudf/_fuzz_testing/main.py	0.00% <ø> (ø)
python/cudf/cudf/_fuzz_testing/orc.py	0.00% <ø> (ø)
python/cudf/cudf/_fuzz_testing/parquet.py	0.00% <ø> (ø)
python/cudf/cudf/_fuzz_testing/utils.py	0.00% <ø> (ø)
... and 76 more

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[shwina]

rerun tests

[galipremsagar]

LGTM

[galipremsagar]

rerun tests

[kkraus14]

@gpucibot merge

[kkraus14]

@gpucibot merge