Add __len__ and __getitem__ methods to PulseSequence

[thangleiter]

This allows for things like

for seg in pulse_sequence:
    do_something_with(seg)

and hence easily investigate parts (time slices) of sequences.

[codecov]

Codecov Report

Merging #67 (d43e184) into master (8bf6159) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master      #67   +/-   ##
=======================================
  Coverage   97.30%   97.31%           
=======================================
  Files           9        9           
  Lines        2229     2237    +8     
  Branches      502      503    +1     
=======================================
+ Hits         2169     2177    +8     
  Misses         26       26           
  Partials       34       34           

Impacted Files	Coverage Δ
filter_functions/pulse_sequence.py	97.66% <100.00%> (+0.02%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 8bf6159...d43e184. Read the comment docs.