Assert that extension size is divisible by 16

[azraq27]

The current implementation of esize makes an @assert check that the size is not divisible by 16, which I believe is the opposite of what is intended.

Minimal example:

dset = niread(fname)
ex = dset.extensions[1]
NIfTI.esize(ex)

AssertionError: NIfTIExtension has innapropriate size. See docstrings for more details.

[codecov]

Codecov Report

Merging #62 (bdad34e) into master (06d4ce3) will not change coverage.
The diff coverage is 0.00%.

@@           Coverage Diff           @@
##           master      #62   +/-   ##
=======================================
  Coverage   52.27%   52.27%           
=======================================
  Files           6        6           
  Lines         549      549           
=======================================
  Hits          287      287           
  Misses        262      262           

Impacted Files	Coverage Δ
src/extensions.jl	15.78% <0.00%> (ø)

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