Fix dl2 naming

[rlopezcoto]

I open this one apart from #344 to see if everybody agrees. I'm not sure why in the dl2 file naming we kept dl1, as in:
dl2_dl1_LST-1...

I propose to eliminate the dl1 part for the future naming of dl2 files

[codecov]

Codecov Report

Merging #345 into master will not change coverage.
The diff coverage is 50.00%.

@@           Coverage Diff           @@
##           master     #345   +/-   ##
=======================================
  Coverage   43.67%   43.67%           
=======================================
  Files          69       69           
  Lines        4623     4623           
=======================================
  Hits         2019     2019           
  Misses       2604     2604           

Impacted Files	Coverage Δ
lstchain/scripts/lstchain_dl1_to_dl2.py	0.00% <0.00%> (ø)
lstchain/scripts/tests/test_lstchain_scripts.py	100.00% <100.00%> (ø)

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[rlopezcoto]

Thanks for the approval @morcuended and sorry, but I had to solve some conflicts before merging (need another approval)