[ENH] [REF] [DOC] Convert pulse trains to new format

[mbeyeler]

• Convert old pulse trains to new format (see issue Convert pulse trains to new Stimulus format #171)
• Have all stimuli and pulses use ms as the fundamental time unit (see issue Time should be in milliseconds #170)
• Move pulses into pulses.py: MonophasicPulse, BiphasicPulse, AsymmetricBiphasicPulse
• Move pulse trains into pulse_trains.py: BiphasicPulseTrain, AsymmetricBiphasicPulseTrain
• Fix many edge cases of combining multiple pulse trains into a single Stimulus
• Mark TimeSeries, PulseTrain as deprecated
• Update docs: gallery examples, topic on stimuli

[codecov-io]

Codecov Report

Merging #178 into master will increase coverage by 1.12%.
The diff coverage is 97.27%.

@@            Coverage Diff             @@
##           master     #178      +/-   ##
==========================================
+ Coverage   87.95%   89.08%   +1.12%     
==========================================
  Files          57       59       +2     
  Lines        4459     4901     +442     
==========================================
+ Hits         3922     4366     +444     
+ Misses        537      535       -2     

Flag	Coverage Δ
#unittests	89.08% <97.27%> (+1.12%)	⬆️

Impacted Files	Coverage Δ
pulse2percept/models/nanduri2012.py	100.00% <ø> (ø)
pulse2percept/stimuli/pulses.py	90.90% <90.90%> (ø)
pulse2percept/stimuli/pulse_trains.py	92.24% <92.00%> (-3.45%)	⬇️
pulse2percept/stimuli/base.py	97.66% <98.91%> (+0.95%)	⬆️
pulse2percept/stimuli/tests/test_pulses.py	99.53% <99.53%> (ø)
pulse2percept/implants/base.py	94.92% <100.00%> (ø)
pulse2percept/models/base.py	94.93% <100.00%> (+5.06%)	⬆️
pulse2percept/models/tests/test_base.py	98.61% <100.00%> (+0.31%)	⬆️
pulse2percept/models/tests/test_beyeler2019.py	100.00% <100.00%> (ø)
pulse2percept/models/tests/test_nanduri2012.py	100.00% <100.00%> (ø)
... and 8 more

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