Create basic ddi parser

[00krishna]

This PR concentrates on developing a basic DDI file parser for an IPUMs extract. The DDI file provides all of the IPUMs and variable metadata, which is necessary for importing the data file itself. In the basic IPUMs download, there is no metadata in the data file.

Included items:

• created a new folder called parsers and added ddi_parser.jl file.
• created DDIInfo and DDIVariable objects to hold metadata for the extract.
• created a new function parse_ddi that takes in a DDI XML file and return a DDI object. The details of the DDI object may be updated later.
• added export and includes to the IPUMS.jl file.
• added a testdata subfolder to the test folder, and include some small test files.
• added 2 test cases to the runtests.jl file.

TODO items:

• Extract the categorical variable metadata from the DDI file.

[codecov]

Codecov Report

Attention: Patch coverage is 98.05825% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 20.26%. Comparing base (12e8c49) to head (9bd0d2f).
Report is 2 commits behind head on main.

Files	Patch %	Lines
src/parsers/ddi_parser.jl	97.97%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##            main      #13       +/-   ##
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+ Coverage   1.19%   20.26%   +19.07%     
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  Files         25       26        +1     
  Lines        420      523      +103     
==========================================
+ Hits           5      106      +101     
- Misses       415      417        +2     

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[TheCedarPrince]

This looks great! Left some comments as a first pass -- mostly to simplify some things and to document/organize various parts of the code. I'll test out the API to see how it feels as well. Thanks @00krishna !

[TheCedarPrince]

Really great stuff here -- the docstrings are fantastic as are the examples within them! Left some feedback and thoughts.

What else are you planning to do with this PR? I am almost wondering if we could make the package extensions a separate PR as well as definitely making the way the parsed information gets returned into a separate PR. What do you think @00krishna ?

[TheCedarPrince]

Oh also, do you know why Documenation is failing in the CI @00krishna ? Finally, in the PR, apparently the argumenterrors are not being code covered. Could you perhaps investigate why that is? Link here: https://app.codecov.io/gh/JuliaHealth/IPUMS.jl/pull/13?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaHealth

[TheCedarPrince]

Ah wait -- I broke documentation. 🤦‍♂️

[00krishna]

Really great stuff here -- the docstrings are fantastic as are the examples within them! Left some feedback and thoughts.

What else are you planning to do with this PR? I am almost wondering if we could make the package extensions a separate PR as well as definitely making the way the parsed information gets returned into a separate PR. What do you think @00krishna ?

Yeah, we can create a separate PR for the package extensions stuff. That is not too bad. I think I am done with this PR, as I have the parsing working, and I also created a simple output that shows the captured metadata. The next PR will actually import the datafile into a dataframe--based upon the parsing info.