Add support for CCITT-Group4 decoding

[rlars]

I created an implementation which does work also with some larger files I tested (10k x 10k pixels) and compared these with the uncompressed output from GIMP. Would you want to integrate this?

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 66.48352% with 61 lines in your changes missing coverage. Please review.

Project coverage is 88.63%. Comparing base (ff965a6) to head (3dcb0e7). Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
src/compression_t6.jl	66.28%	59 Missing :warning:
src/ifds.jl	83.33%	1 Missing :warning:
src/layout.jl	0.00%	1 Missing :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #190      +/-   ##
==========================================
- Coverage   93.33%   88.63%   -4.71%     
==========================================
  Files          15       16       +1     
  Lines        1186     1364     +178     
==========================================
+ Hits         1107     1209     +102     
- Misses         79      155      +76     

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