Update dtype on DataColumn object instead of DataTable

[thehomebrewnerd]

Closes #170

Moves the _update_dtype method to the DataColumn object, so that instantiating a column directly will modify the series dtype as needed depending on the LogicalType.

Updated DataTable._update_columns method to make sure dataframe stays in sync with column series data since calling astype on a series returns a new series object.

Updated various test as a result of these changes and added new DataColumn tests to confirm physical types are changed as expected.

[codecov]

Codecov Report

Merging #233 into main will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #233   +/-   ##
=======================================
  Coverage   99.95%   99.95%           
=======================================
  Files          22       22           
  Lines        2301     2307    +6     
=======================================
+ Hits         2300     2306    +6     
  Misses          1        1           

Impacted Files	Coverage Δ
woodwork/data_column.py	100.00% <100.00%> (ø)
woodwork/data_table.py	100.00% <100.00%> (ø)
woodwork/tests/data_column/test_data_column.py	100.00% <100.00%> (ø)

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