New Analog API Proposal

***averaging stuff***
new analog init function
- don't worry about speeding up analog loop for now (do that separately)
- will take the number of samples for averaging as an argument
  - make old init function a wrapper for this one that uses a default value
  - will not handle multiple buffer lengths
- initialize averaging buffers
- bitfield (or some other way) to determine which ADC ports to average

new "get" functions
- take an argument to specify average or immediate value
- if averaging, perform division on running sum
- deprecate the old get

#define some new error codes
- out of range
- trying to average, but not enough history values

in the interrupt
- fill the ring buffer with the newest reading and add the current readings for the average
- don't do the divide for the averaging - do this in get

update rangefinder code accordingly so that people can get averaged rangefinders

***new analog interrupt***
init function
-takes a bitfield of different analog fields to read
-analog loop only reads those analog fields

new "get" function
-keep it fresh - return an error if somebody tries to read an old value twice

function to update the bitfield dynamically

eliminate bom_refresh - put bom refresh in the analog loop when the bom bit is asserted in the bitfield

#define some new error codes
- analog port not initialized yet
- not reading from that analog - bit not asserted in bitfield
- reading is not fresh - this particular reading has already been read

no need to start and stop the loop anymore - deprecate the old functions
- to do this, user should set all bits in bitfield to 0

issues to consider:
- concurrency issue if user disables a port that is currently being read (is this serious?)