Add consistent fractional share handling

[jdfergason]

Not all brokerages support fractional shares and the ones that do support different minimum amounts. E.g. fidelity supports fractional shares for all NYSE and NASDAQ securities in increments of .001, IBKR supports US and European stocks in increments of .0001, Charles Schwab only supports stocks in the SP500 with a minimum investment of $5 and a maximum of $50,000.

A portfolio setting should allow users to enable / disable fractional share trading and set the minimum increment (i.e. .1, .01, .001, .0001, etc) stock quantities below these minimums would be rounded to the nearest minimum amount.

The difference between .001 and .002 is neglible for most stocks. E.g., a stock trading at $500/share is $0.50/per .001 fraction. However there are a handful of stocks, like BRK/A that have a more significant difference (BRK/A is $400/per .001 fraction).

[jdfergason]

Proposal:

1. In portfolio settings enable minimum share holding (1, .1, .01, .001, .0001, and .00001)
2. for share requests outside of the minimum share holding round down to the nearest minimum share holding, e.g. if the minimum share holding is .1 then .456 is rounded to .4 shares