downloadImage fix for capatitive sensors 192 x 192 pixels

[tom-van]

The change adds some level of the sensor size autodetection to downloadImage() based on the dictionary of the known sensor sizes. Also a setter function setSensorSize() is added for manual setting of sensor size (e.g. before use of uploadImage() if it will be implemented in the future)

downloadImage() internal storage imageData is changed from 2 dimensional list to a simple list as there is no point in row/column division - data packets have a configurable size which not necessarily corresponds to image rows.

[tom-van]

I tested the PR with GROW R503 http://www.zjgrow.com/grow-r503-new-circular-round-two-color-ring-indicator-led-control-dc33v-mx10-6pin-capacitive-fingerprint-module-sensor-scanner-p2112363.html