Add pandas_dtype to LogicalType

[thehomebrewnerd]

This PR implements two changes in preparation for performing dataframe dtype conversions:

• Adds pandas_dtype to each logical type, representing the string alias for the underlying dtype that should be associated with the LogicalType
• Removes the dtype attribute from DataColumn and replaces it with a property that returns the dtype of the underlying series.

Closes #84

[codecov]

Codecov Report

Merging #85 into main will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main      #85   +/-   ##
=======================================
  Coverage   99.13%   99.14%           
=======================================
  Files          12       12           
  Lines         578      582    +4     
=======================================
+ Hits          573      577    +4     
  Misses          5        5           

Impacted Files	Coverage Δ
data_tables/data_column.py	100.00% <100.00%> (ø)
data_tables/logical_types.py	96.49% <100.00%> (+0.19%)	⬆️

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[gsheni]

@thehomebrewnerd Should we write some tests that verify these dtypes are properly set when we do the conversions? OR will that be in a follow-up MR?

[thehomebrewnerd]

@thehomebrewnerd Should we write some tests that verify these dtypes are properly set when we do the conversions? OR will that be in a follow-up MR?

I think that should be in a follow up MR. As of now since there is no code to change the dtype, I don't think we can implement those tests. But, yes, I agree we should test that out for all these to make sure it works as expected once the conversion code is in place - we will confirm that the series dtype matches the type specified here.