To be able to compute E-UTRAN IP Throughput amari.drb uses Sampler which
detects and extracts separate transmission samples from 100Hz log of
ue_get[stats]. The Sampler, in turn uses help from _BitSync for correct
operations because Amarisoft LTEENB updates counters for dl_total_bytes
and dl_tx at different times, and _BitSync synchronizes streams of those
updates in time. _BitSync itself works by correlating amount of
transmitted data (tx_bytes) and transport blocks (#tx) and shifting some
amount of #tx to previous frame based on the correlation. See d102ffaa
(amari.drb: Start of the package) for details.

This works ok for configurations with 1 cell, but does not work out of the box
for multicell configurations because #tx is per-cell value, while e.g.
dl_total_bytes is reported by LTEENB only as a value aggregated over all cells.
That's why original implementation in d102ffaa had an assert that the
number of cells an UE is associated with is 1.

-> Implement custom filter that splits overall tx_bytes into per-cell
   parts via heuristic based on reported per-cell bitrates to workaround
   that: the more cell bitrate is the more is the part of tx_bytes that
   gets associated to this cell.

In basic implementation the heuristic would be to divide tx_bytes as

	tx_bytes(cell) = tx_bytes·β/Σcells(β)		; β is bitrate of a cell

but given that for every frame _BitSync works by computing things based
on neighbour frame as well we do it as

	tx_bytes(cell) = tx_bytes·(β₁+β₂)/Σcells(β₁+β₂)

This should make the heuristic a bit more stable.

This patch comes with tx_bytes splitter filter itself only. In the next
patch we will use _CTXBytesSplitter to implement multicell-awareness for
_BitSync.