Minor changes to support zfiles tests

[kkappler]

@kujaku11 I made some minor changes in the previous two commits that broke z file tests.
Tests now run fine with the very minor changes here.

These are related to the two things I added:

1. Read/Write processing type from header of z file (if present)
2. Now packing decimation dict (and passing it to TF) on z file read-in.
   PTAL

[codecov-commenter]

Codecov Report

Patch coverage: 79.06% and project coverage change: +0.45% 🎉

Comparison is base (3fabc9c) 81.86% compared to head (f602072) 82.31%.
Report is 10 commits behind head on fcs.

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Additional details and impacted files

@@            Coverage Diff             @@
##              fcs     #163      +/-   ##
==========================================
+ Coverage   81.86%   82.31%   +0.45%     
==========================================
  Files         269      269              
  Lines       19502    19547      +45     
==========================================
+ Hits        15966    16091     +125     
+ Misses       3536     3456      -80     

Files Changed	Coverage Δ
...ata/transfer_functions/io/emtfxml/metadata/data.py	70.21% <0.00%> (ø)
.../transfer_functions/io/emtfxml/metadata/helpers.py	86.41% <0.00%> (-1.09%)	⬇️
tests/tf/io/zmm/test_tf_read_zmm.py	98.78% <ø> (ø)
tests/tf/io/zmm/test_tf_read_zss.py	98.07% <ø> (ø)
..._metadata/transfer_functions/io/emtfxml/emtfxml.py	65.63% <10.00%> (-0.27%)	⬇️
mt_metadata/transfer_functions/core.py	68.28% <50.00%> (+0.15%)	⬆️
mt_metadata/transfer_functions/io/zfiles/zmm.py	66.87% <66.66%> (+0.13%)	⬆️
...ions/processing/fourier_coefficients/decimation.py	73.00% <81.25%> (+28.51%)	⬆️
mt_metadata/utils/mttime.py	83.11% <100.00%> (+0.66%)	⬆️
tests/test_mttime.py	98.56% <100.00%> (+0.02%)	⬆️
... and 1 more

... and 2 files with indirect coverage changes

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