Add after-tax-yield calculations

[bondgeek]

Two scenarios: 1) taxable bond 2) tax-exempt bond subject to Accrued Market Discount taxation

• any market discount, i.e. the discount in excess of the OID, is subject taxation as ordinary income at maturity. The accrued portion is taxed if the bond is sold prior to maturity. If the market discount is "de minimus" defined as less than 1/4 point per integer number of years to maturity, the accrued discount is treated as a capital gain.

BondBase class variables:

• bond interest tax-exempt flag

Settings variables:

• capital gains tax rate
• income tax rate
• deminimus points per year

[bondgeek]

Not sure why this was closed. Add after-tax calculations to BondBase in Cython code.