Add option to oversample thermistor ADC values.

This improves the stability of PID control by reducing the effect of quantization
on the operation of the D term.

Also switched adc_value_to_temperature() to use float math instead of unnecessarily
precise double calculations.

http://koti.kapsi.fi/jpa/stuff/other/therm_test/report.html (current edge / new default is first image, pull request + config is last image)
This feature is off by default, set temperature_control.hotend.oversample_freq to e.g. 1000 Hz to enable it.
By oversampling the ADC value, we get more accurate and less quantized temperature readings.
This in turn makes the PID D-term behave better and improves temperature stability.
The effect is largest at high temperatures (>200 C), where the steps are larger with 100K NTC.
default should not be 0. unless you later test for zero and not set a slowtimer with a frequency of 0