Allow different target volatilities for each asset

[danilogalisteu]

This change allows different target volatilities for each asset, by passing a dict of values instead of a single one.

Fixes #307

MWE:

import bt

data = bt.get(["spy", "qqq", "gld"])

vol_target_dict = {
    "spy": 0.07,
    "qqq": 0.05,
    "gld": 0.03,
}

s = bt.Strategy('s', [bt.algos.RunMonthly(),
                      bt.algos.SelectAll(),
                      bt.algos.WeighInvVol(),
                      bt.algos.TargetVol(vol_target_dict),
                      bt.algos.Rebalance()])

b = bt.Backtest(s, data)
res = bt.run(b)
res.display()
res.plot()
res.plot_security_weights('s')

[codecov-commenter]

Codecov Report

Merging #306 (3fc29d4) into master (03e89d3) will increase coverage by 0.04%.
The diff coverage is 50.00%.

@@            Coverage Diff             @@
##           master     #306      +/-   ##
==========================================
+ Coverage   46.24%   46.29%   +0.04%     
==========================================
  Files           4        4              
  Lines        1905     1903       -2     
  Branches      375      376       +1     
==========================================
  Hits          881      881              
+ Misses        972      970       -2     
  Partials       52       52              

Impacted Files	Coverage Δ
bt/algos.py	85.94% <50.00%> (+0.19%)	⬆️

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