Add FutureWarning for default filesetter external engines

[sroet]

this adds a FutureWarning to let users know that in OPS 2.0 the default FileSetter will become random string

This was part of the discussion on #1101

[codecov]

Codecov Report

Merging #1102 (d342f02) into master (dd2cad5) will increase coverage by 0.00%.
The diff coverage is 83.33%.

@@           Coverage Diff           @@
##           master    #1102   +/-   ##
=======================================
  Coverage   81.75%   81.75%           
=======================================
  Files         142      142           
  Lines       15611    15612    +1     
=======================================
+ Hits        12763    12764    +1     
  Misses       2848     2848           

Impacted Files	Coverage Δ
openpathsampling/engines/external_engine.py	84.39% <80.00%> (ø)
openpathsampling/deprecations.py	100.00% <100.00%> (ø)

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[dwhswenson]

LGTM, except the one spelling change I'll enforce.