Type inference and improved meta layer

[dcmoura]

This PR adds:

• Inference of basic data type (int, float, complex) for columns in CSV files. Resilient to NULLs. Defaults as string.
• cols var/translation gets list of columns' values for the current record (after type casting)
• _values var holds the list of columns' values for the current record (before type casting). Should be equivalent to cols except on CSVs.
• row var/translation returns a dictionary with the current row (col names as keys, cols as values)
• _names var holds the list of column names

Minor bugs and improvements (see list of commits).

This PR will expire in 24 hours :-)
I will merge to master by then if there's no feedback.

[codecov[bot]]

Codecov Report

Merging #47 (436e3e7) into master (f1163f2) will increase coverage by 0.17%. The diff coverage is 98.03%.

:exclamation: Current head 436e3e7 differs from pull request most recent head 355e2df. Consider uploading reports for the commit 355e2df to get more accurate results

@@            Coverage Diff             @@
##           master      #47      +/-   ##
==========================================
+ Coverage   95.87%   96.04%   +0.17%     
==========================================
  Files          10       10              
  Lines        1018     1063      +45     
==========================================
+ Hits          976     1021      +45     
  Misses         42       42              

Impacted Files	Coverage Δ
spyql/processor.py	96.44% <97.77%> (+0.52%)	:arrow_up:
spyql/writer.py	96.18% <100.00%> (+0.15%)	:arrow_up:

---

Continue to review full report at Codecov.

Legend - Click here to learn more Δ = absolute <relative> (impact), ø = not affected, ? = missing data Powered by Codecov. Last update f1163f2...355e2df. Read the comment docs.