Allow adjustment to first-level spread in balanced strategy

[Reidmcc]

Desired Behavior

When trading on a centralized exchange with percentage fees, a larger inner spread is necessary compared to the SDEX for similar profitability. You can of course set Kelp's balanced spread high enough, however doing so will space your subsequent levels very far apart, which is not necessarily desirable.

For example, if you want to use the balanced strategy with the behavior it executes with spread set to 0.002, but trade on an exchange with 0.0015/trade fees, you can't account for those fees and maintain the spread=0.002 behavior outside the inner spread. To effectively have an inner spread of 0.002, accounting for fees, you would have to set Kelp's spread to 0.005, spacing your subsequent levels farther apart than desired.

To address this, I want to be able to adjust the inner spread on the balanced strategy, in addition to setting the overall spread parameter.

Impact

The current balanced strategy cannot effectively execute the strategy in a way that mirrors its behavior on the SDEX on a fee exchange. This feature would allow it to do so.

Feature Suggestion

We can achieve this by adding a spread pad parameter that the balanced strategy uses to adjust its effective center price. This would retain the strategy's normal behavior by adjusting only the reference point, not price or amount calculations.

Specification

Pull request incoming.