Add support for single gain telescope to the pulse template from eventsource

[gabemery]

closes #1058

This was tested on current test files temporary using telescopes [1, 6] instead of [1, 2] in the failing test since LST2-4 are in the current file.
Failure behaviour in #1058 was observed before the fix.

[codecov]

Codecov Report

Base: 71.72% // Head: 71.72% // Increases project coverage by +0.00% 🎉

Coverage data is based on head (d17fb79) compared to base (3a04678).
Patch coverage: 80.00% of modified lines in pull request are covered.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1061   +/-   ##
=======================================
  Coverage   71.72%   71.72%           
=======================================
  Files         120      120           
  Lines       11316    11320    +4     
=======================================
+ Hits         8116     8119    +3     
- Misses       3200     3201    +1     

Impacted Files	Coverage Δ
lstchain/data/normalised_pulse_template.py	98.68% <80.00%> (-1.32%)	⬇️

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