Change the application of 'maxsig' threshold parameter

[mcgarth]

In this PR I address the Github issue https://github.com/GeoscienceAustralia/PyRate/issues/247

• Move Stack Rate masking to a standalone function pyrate.core.stack.mask_rate, applied during the merge step and add unit tests.
• Skip Stack Rate masking if threshold parameter maxsig = 0.
• Provide log message indicating the percentage of pixels masked by pyrate.core.stack.mask_rate.
• Refactor pyrate.core.stack module; expose two functions in documentation: i) single pixel stacking algorithm, and ii) loop function for processing full ifg array.
• Refactor pyrate.merge script; remove duplicated code and create reusable generic functions.
• Updated test and dev requirements.

[codecov-commenter]

Codecov Report

:exclamation: No coverage uploaded for pull request base (develop@d340c41). Click here to learn what that means. The diff coverage is 92.24%.

@@            Coverage Diff             @@
##             develop     #269   +/-   ##
==========================================
  Coverage           ?   90.77%           
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  Files              ?       25           
  Lines              ?     3383           
  Branches           ?      567           
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  Hits               ?     3071           
  Misses             ?      205           
  Partials           ?      107           

Impacted Files	Coverage Δ
pyrate/core/config.py	80.62% <ø> (ø)
pyrate/prepifg.py	99.13% <ø> (ø)
pyrate/merge.py	90.00% <91.25%> (ø)
pyrate/core/stack.py	97.22% <93.10%> (ø)
pyrate/configuration.py	100.00% <100.00%> (ø)
pyrate/core/aps.py	85.25% <100.00%> (ø)
pyrate/process.py	90.19% <100.00%> (ø)
pyrate/core/gamma.py	97.93% <0.00%> (ø)
pyrate/core/orbital.py	95.70% <0.00%> (ø)
... and 22 more

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[mcgarth]

Thanks for the reviews! Merging, but will look in to the error "Failed to compute statistics, no valid pixels found in sampling"