input/csv: move channel creation to after format parsing

input/csv: move channel creation to after format parsing

The previous implementation incompletely handled arbitrary data type
oders in mixed signal input files. Rearrange the logic such that all
format specs get parsed first, then all channel creation details get
determined, then all channels get created. It appears to be essential
that all logic channels get created first, resulting in index numbers
starting at 0 and addressing the correct position in bitfields. The
analog channels get created after all logic channels exist. Adjacent
number ranges for channel types also results in more readable logic in
other locations.

This was tested with -I csv:column_formats=t,2a,l,a,x4,a,-,b3 example
data, and works as expected.