DL3 energy-dependent alpha cut

[SeiyaNozaki]

This PR implements energy-dependent alpha cut for the IRF/DL3 production.
The same solution as energy-dependent g/h cuts is basically used for the energy-dependent alpha cut.
(create a separate temporary header to save energy-dependent alpha cuts in IRF)
Since the alpha cut is already applied to observational DL3 data, there is no need to modify gammapy.

[codecov]

Codecov Report

Merging #942 (90a93b7) into master (33a6355) will increase coverage by 0.15%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #942      +/-   ##
==========================================
+ Coverage   85.41%   85.56%   +0.15%     
==========================================
  Files          78       78              
  Lines        6501     6555      +54     
==========================================
+ Hits         5553     5609      +56     
+ Misses        948      946       -2     

Impacted Files	Coverage Δ
lstchain/io/event_selection.py	100.00% <100.00%> (ø)
lstchain/io/tests/test_eventselection.py	100.00% <100.00%> (ø)
lstchain/tools/lstchain_create_dl3_file.py	92.59% <100.00%> (+2.59%)	⬆️
lstchain/tools/lstchain_create_irf_files.py	93.95% <100.00%> (+0.54%)	⬆️
lstchain/tools/tests/test_tools.py	100.00% <100.00%> (ø)

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[chaimain]

Sorry for the late review.
It looks good to me in general.