rework ifd handling and deprecate directly accessing ifds

[tlnagy]

fixes #44

img.ifds is replaced with ifds(img).

[codecov-commenter]

Codecov Report

Patch coverage: 85.71% and project coverage change: -0.06% ⚠️

Comparison is base (76b89f9) 90.27% compared to head (c1801d4) 90.22%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #110      +/-   ##
==========================================
- Coverage   90.27%   90.22%   -0.06%     
==========================================
  Files          13       13              
  Lines         761      767       +6     
==========================================
+ Hits          687      692       +5     
- Misses         74       75       +1     

Files Changed	Coverage Δ
src/TiffImages.jl	100.00% <ø> (ø)
src/types/lazy.jl	89.85% <80.00%> (ø)
src/types/common.jl	86.95% <83.33%> (-1.28%)	⬇️
src/types/dense.jl	94.44% <100.00%> (ø)

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.