adjust sample time shorter for quicker response when turning pump on

measured loop time was around 500 ms, so trying that for sample time
edit readme
edit and add to readme.md, cleanup and fine-tune EC155Controller.ino
set different p_gain for idle vs. pump on, retune PID, bugfix

bugfix: heater_pwm was a uint8_t but would roll over when there were high
	or negative errors. changed it to int16_t to handle larger or
	negative numbers.
various small improvements and refactorings

* print "Initializing..." and check for NaN from the thermocouple while initializing in setup()
* initialize p_gain instead of calculating it from p_band every loop
* set set_temp[0] to 0 when potentiometer is fully CCW
* don't reset i_term when crossing 0 error (that made it oscillate because the needed bias got deleted when it got to setpoint)
* display the time the pump is on in seconds instead of minutes:seconds
* rename some constants and variables
compute PID output only once per sample, integrate error and integral weight together
change heater control to be on 100% when pump is on, retune PID to be less agressive
add derivative term, initial PID tuning
add integration, now it is a PI controller
code cleanup and bug fixes
modify SSR holder to 3d print more reliably
add 3d printable SSR holder
move schematic to its own directory
initial readme.md
update schematic: change pump ssr from zero-cross to random-fire
make corrections and clarifications to schematic
change integer variables to exact-width
finish schematic