Use *float for SimpleEWMA

[andig]

These properties let it use less memory. It will also behave differently when it's equal to zero, which is assumed to mean uninitialized, so if a value is likely to actually become zero over time, then any non-zero value will cause a sharp jump instead of a small change.

The disadvantages for SimpleEWMA like the sensitivity for 0 values could be solved by using *float64 for storing the value internally. Could do a PR if desired.