shrink int to auto-map to int8

[lawrence-mbf]

Fixes #486

Motivation

Allows auto-shrink to all integer types.

How to test the behavior?

tests should be successful when exported by MatNWB and read in by PyNWB.

Checklist

• Have you ensured the PR description clearly describes the problem and solutions?
• Have you checked to ensure that there aren't other open or previously closed Pull Requests for the same change?
• If this PR fixes an issue, is the first line of the PR description fix #XX where XX is the issue number?

[codecov]

Codecov Report

Merging #523 (31529a5) into master (461c5c4) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #523   +/-   ##
=======================================
  Coverage   87.89%   87.89%           
=======================================
  Files         129      129           
  Lines        5461     5461           
=======================================
  Hits         4800     4800           
  Misses        661      661           

Files Changed	Coverage Δ
+file/mapType.m	94.28% <100.00%> (ø)

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