Review the meaning of overrun in the docs

[dotdoom]

Overrun is actually not overflow but a signal that a new value was written to the register before the previous was read (i.e. reading rate is too slow). An appropriate error/warning needs to be generated.

Additionally, to improve reading rate we should not read configuration registers while reading the output; last set config option should be cached. Might also consider simplifying the methods for acquisition, e.g. disable "number of acquisitions" in continuous mode.