add adc_histogram example

[ericfont]

I find your library very useful. I wanted to analyze how noisy the ADC is under different configurations, so I made this program using your library to plot a histogram of the distribution of results from continuous read. Each possible configuration runs for 1 second and prints the resulting histogram as well as summary statistics.

For example, if I use a stable voltage source going into both ADC inputs, then my program prints this output (full file: results_both_all-configs.txt) for the most noisy configuration of ADC settings:

ADC0 config: BitDepth=12, averaging=1, ADC_CONVERSION_SPEED=ADACK_20, ADC_SAMPLING_SPEED=VERY_HIGH_SPEED.
716226 measurements, at a rate of 1.396 microsPerMeasurement or 716.23 kHz.
bin    [index]     count    percent of total measurements
0.2371 [971]:        126 = 0.02%
0.2374 [972]:        712 = 0.10%
0.2376 [973]:        282 = 0.04%
0.2379 [974]:        637 = 0.09%
0.2381 [975]:        117 = 0.02%
0.2383 [976]:       1846 = 0.26%
0.2386 [977]:       2306 = 0.32%
0.2388 [978]:      11262 == 1.57%
0.2391 [979]:       5042 = 0.70%
0.2393 [980]:       8019 == 1.12%
0.2396 [981]:       7939 == 1.11%
0.2398 [982]:       9151 == 1.28%
0.2400 [983]:      42873 ====== 5.99%
0.2403 [984]:      42546 ====== 5.94%
0.2405 [985]:     123943 ================== 17.31%
0.2408 [986]:     225235 ================================ 31.45%
0.2410 [987]:     117773 ================= 16.44%
0.2413 [988]:      39538 ====== 5.52%
0.2415 [989]:      11303 == 1.58%
0.2418 [990]:       8007 == 1.12%
0.2420 [991]:       1481 = 0.21%
0.2422 [992]:       6437 = 0.90%
0.2425 [993]:       6516 = 0.91%
0.2427 [994]:       6568 = 0.92%
0.2430 [995]:       4092 = 0.57%
0.2432 [996]:       4530 = 0.63%
0.2435 [997]:       2530 = 0.35%
0.2437 [998]:       2160 = 0.30%
0.2440 [999]:       1859 = 0.26%
0.2442 [1000]:       1163 = 0.16%
0.2444 [1001]:       1294 = 0.18%
0.2447 [1002]:       1720 = 0.24%
Summary:
range:  0.019536020 over 80 bins from [962] to [1041]
mode:   0.240781441 [986]
mean:   0.240923390
var:    0.000001651
stdDev: 0.001285070

I'm making this PR cause others might find this useful as a test example and to help them evaluate what noise-vs-speed tradeoff works best for their application.