fixed key error when adding multiple sector constraints

[andyherfer]

I wanted to add multiple sector constraints, for example:

mapper1 = {
     "AAPL": "ESG",
     "PG": "NOTESG"
     . 
     .
     .
   }
lower1 = {
    "ESG": 0.1
}
upper1 = {
    "NOTESG": 0.1
}

ef.add_sector_constraint(mapper1, lower1, upper1)

mapper2 = {
     "AAPL": "TECH",
     "PG": "DEFENSIVE"
     . 
     .
     .
   }
lower2 = {
    "DEFENSIVE": 0.1
}
upper2 = {
    "TECH": 0.2
}

ef.add_sector_constraint(mapper2, lower2, upper2)

Where the main problem was that some assets belonged to more than one sector, and wanted to mix different sector classifications, such as debt can be subdivided between corporate and governmental.

When adding the second constraint, it looks for all registered tickers, where some may not be present in the second mapper, thus by changed the [] dict indexing notation for a .get notation, a None is returned, and therefore not added to the optimization constraint sum in:

        for sector in sector_upper:
            is_sector = [sector_mapper.get(t) == sector for t in self.tickers]
            self.add_constraint(lambda w: cp.sum(w[is_sector]) <= sector_upper[sector])
        for sector in sector_lower:
            is_sector = [sector_mapper.get(t) == sector for t in self.tickers]
            self.add_constraint(lambda w: cp.sum(w[is_sector]) >= sector_lower[sector])

[codecov-commenter]

Codecov Report

Patch coverage: 100.00% and project coverage change: -0.12 :warning:

Comparison is base (6f699fd) 99.42% compared to head (3feb53a) 99.30%.

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #531 +/- ## ========================================== - Coverage 99.42% 99.30% -0.12% ========================================== Files 33 33 Lines 5072 5072 ========================================== - Hits 5043 5037 -6 - Misses 29 35 +6 ``` | [Impacted Files](https://codecov.io/gh/robertmartin8/PyPortfolioOpt/pull/531?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None) | Coverage Δ | | |---|---|---| | [pypfopt/base\_optimizer.py](https://codecov.io/gh/robertmartin8/PyPortfolioOpt/pull/531?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None#diff-cHlwZm9wdC9iYXNlX29wdGltaXplci5weQ==) | `97.54% <100.00%> (ø)` | | ... and [1 file with indirect coverage changes](https://codecov.io/gh/robertmartin8/PyPortfolioOpt/pull/531/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None)

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[88d52bdba0366127fffca9dfa93895]

Thanks you for the fix.