add resolution field to SingleTimeSeries

[jd-lara]

Adds resolution field to SingleTimeSeries and avoids making the calculation on the fly. closes #236

[jd-lara]

cc. @andrewrosemberg this resolves the issue in IS with problems horizon = 1

[codecov[bot]]

Codecov Report

Merging #237 (c00173f) into master (c995be8) will decrease coverage by 0.06%. The diff coverage is 83.33%.

@@            Coverage Diff             @@
##           master     #237      +/-   ##
==========================================
- Coverage   75.40%   75.33%   -0.07%     
==========================================
  Files          43       43              
  Lines        3122     3122              
==========================================
- Hits         2354     2352       -2     
- Misses        768      770       +2     

Flag	Coverage Δ
unittests	75.33% <83.33%> (-0.07%)	:arrow_down:

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/single_time_series.jl	66.66% <83.33%> (-0.36%)	:arrow_down:
src/utils/utils.jl	61.18% <0.00%> (-0.43%)	:arrow_down:
src/utils/lazy_dict_from_iterator.jl	96.15% <0.00%> (-0.15%)	:arrow_down:
src/time_series_parameters.jl	94.84% <0.00%> (-0.06%)	:arrow_down:

[andrewrosemberg]

LGTM