input/csv: robustness nits in column format dispatching

input/csv: robustness nits in column format dispatching

The previous implementation open coded type checks by comparing an enum
value against specific constants. Which was especially ugly since there
are multiple types which all are logic, and future column types neither
get ignored nor have channels associated with them.

Improve readability and robustness by adding helpers which check classes
(ignore, logic, analog) instead of the multitude of -/x/o/b|l/a variants
within the classes.

Also comment on the order of channel creation, and how to improve it.