Provide default table object when not parsing into standard format

[alecloudenback]

Closes #107

• return generic datatype for non-standard tables
• decouple parsing logic to not assume everthing is a select or ultimate mortality table

improve generic data returned by:

• add usage warning via @info

• refactor interim steps to make fewer assumptions about the contents of the table (e.g. parse_2D_table assumes indices are issue_age and duration)

• iterate on generic table API to get to a happy spot

• parse table metadata for display methods and/or value index information

• how to handle non-contiguous indices? idea: just be missing for missing values
  other todo:

• docuemnt in API which parts are stable, experimental

• add test cases

[codecov]

Codecov Report

Merging #115 (55cab4a) into master (171fa69) will decrease coverage by 67.24%.
The diff coverage is 87.09%.

@@             Coverage Diff             @@
##           master     #115       +/-   ##
===========================================
- Coverage   90.16%   22.92%   -67.25%     
===========================================
  Files           9        8        -1     
  Lines         366      349       -17     
===========================================
- Hits          330       80      -250     
- Misses         36      269      +233     

Impacted Files	Coverage Δ
src/XTbML.jl	66.08% <87.09%> (+0.97%)	⬆️
src/get_SOA_table.jl	0.00% <0.00%> (-100.00%)	⬇️
src/MortalityTable.jl	0.00% <0.00%> (-98.19%)	⬇️
src/parameterized_models.jl	0.00% <0.00%> (-98.06%)	⬇️
src/CSV.jl	0.00% <0.00%> (-97.73%)	⬇️
src/life_expectancy.jl	0.00% <0.00%> (-90.91%)	⬇️
src/death_distribution.jl

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[MatthewCaseres]

I am thinking about doing no modifications to the existing API, except restrict the tables that can be loaded to ones that we are certain are going to work well. then provide a new API which is a constructor of an XTbML struct.