Make time and pulse duration properties

[thangleiter]

That way we can lazily evaluate them (PulseSequence.t is not needed for concatenation, for instance).

[codecov]

Codecov Report

Merging #61 (b68cab6) into master (5f32bff) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #61      +/-   ##
==========================================
+ Coverage   97.27%   97.30%   +0.02%     
==========================================
  Files           9        9              
  Lines        2203     2223      +20     
  Branches      498      501       +3     
==========================================
+ Hits         2143     2163      +20     
  Misses         26       26              
  Partials       34       34              

Impacted Files	Coverage Δ
filter_functions/pulse_sequence.py	97.64% <100.00%> (+0.31%)	⬆️
filter_functions/numeric.py	94.33% <0.00%> (-0.50%)	⬇️

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