Account for negative inflation in PD controller

[sug0]

From the refactor in #2575:

namada/crates/controller/src/lib.rs

Lines 78 to 81 in 4ed6229

	// TODO: could possibly use I256 instead of Dec here (need to account for
	// negative vals)
	fn compute_inflation_aux(&self, control: Dec) -> Result<Uint, Error> {

[brentstone]

What do you mean by account for negative inflation in PD controller here? Dec can also contain negative values. I put the TODO here in case we wanted to do arithemtic just using integers rather than the Dec type, though I think perhaps we can remove the TODO. These numbers need to be signed since control may be negative.

[sug0]

removed in #3234