fix: make sure netcdf exporters can handle list of timesteps

[RubenImhoff]

This PR ensures that the netcdf exporters can handle a list of timesteps next to just an integers number of timesteps. One of pysteps' functionalities is that you can give it specific output timesteps for which the nowcasts should be stored, but it was not yet possible to store these output timesteps as such in a netCDF. This PR tries to fix that.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 83.52%. Comparing base (1525178) to head (2abd113). Report is 13 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #369      +/-   ##
==========================================
+ Coverage   83.33%   83.52%   +0.18%     
==========================================
  Files         161      159       -2     
  Lines       12370    12575     +205     
==========================================
+ Hits        10309    10503     +194     
- Misses       2061     2072      +11     

Flag	Coverage Δ
unit_tests	83.52% <100.00%> (+0.18%)	:arrow_up:

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