Add support for weighting lambda schedule stages

[lohedges]

This PR adds support for weighting lambda schedule stages, allowing a user to adjust the lambda range for given stages, e.g.:

# A three-stage schedule where the central morph stage is given twice
# the lambda range of the charge-scaling stages.
s = sr.cas.LambdaSchedule()
s.add_morph_stage("decharge", weight=1)
s.add_morph_stage("morph", weight=2)
s.add_morph_stage("recharge", weight=1)

# Stage boundaries: decharge [0, 0.25), morph [0.25, 0.75), recharge [0.75, 1]

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