add observe_once constraint

[lspitler]

This gives the user the ability to observe each target in the target list only once.

It is enabled via the config yaml file:

scheduler:
  constraints:
    observe_once: true

[lspitler]

Related to #466 but that issue is for individual targets whereas the functionality captured here applies to all targets.

[codecov[bot]]

Codecov Report

Merging #467 (170c762) into develop (08e90f1) will increase coverage by 0.03%. The diff coverage is 33.33%.

:exclamation: Current head 170c762 differs from pull request most recent head 33da532. Consider uploading reports for the commit 33da532 to get more accurate results

@@             Coverage Diff             @@
##           develop     #467      +/-   ##
===========================================
+ Coverage    56.13%   56.17%   +0.03%     
===========================================
  Files           58       58              
  Lines         3885     3888       +3     
  Branches       407      408       +1     
===========================================
+ Hits          2181     2184       +3     
  Misses        1604     1604              
  Partials       100      100              

Impacted Files	Coverage Δ
src/huntsman/pocs/utils/huntsman.py	73.13% <33.33%> (-1.87%)	:arrow_down:
src/huntsman/pocs/archive/archiver.py	90.00% <0.00%> (+1.81%)	:arrow_up:

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