Add method field to fillna for fixed width columns

[isVoid]

Closes #1361

• Provides "ffill" and "bfill" fillna methods for Numerical, Datetime, Timedelta and Categorical type column.
• Supports method parameter for Series.fillna and DataFrame.fillna

[codecov]

Codecov Report

Merging #6998 (fd9d49f) into branch-0.18 (ab8c931) will increase coverage by 0.04%.
The diff coverage is 81.03%.

@@               Coverage Diff               @@
##           branch-0.18    #6998      +/-   ##
===============================================
+ Coverage        82.01%   82.06%   +0.04%     
===============================================
  Files               96       96              
  Lines            16338    16399      +61     
===============================================
+ Hits             13400    13458      +58     
- Misses            2938     2941       +3     

Impacted Files	Coverage Δ
python/cudf/cudf/core/column/categorical.py	93.35% <60.00%> (+0.01%)	⬆️
python/cudf/cudf/core/column/string.py	86.48% <75.00%> (-0.10%)	⬇️
python/cudf/cudf/core/frame.py	90.26% <75.00%> (-0.09%)	⬇️
python/cudf/cudf/core/column/timedelta.py	89.16% <87.50%> (-0.38%)	⬇️
python/cudf/cudf/core/column/column.py	88.14% <100.00%> (+0.21%)	⬆️
python/cudf/cudf/core/column/datetime.py	89.42% <100.00%> (+0.53%)	⬆️
python/cudf/cudf/core/column/numerical.py	94.41% <100.00%> (-0.13%)	⬇️
python/cudf/cudf/core/series.py	91.10% <100.00%> (ø)
python/cudf/cudf/io/dlpack.py	95.23% <0.00%> (ø)
... and 5 more

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[isVoid]

rerun tests

[isVoid]

rerun tests