Improve pspm_write_channel

[dominikbach]

Feature Description

Currently, pspm_write_channel, overwrites the last channel of same type if channel_action is 'replace'. However, when channels are defined by numerical index in the calling function, it would be more appropriate to overwrite the input channel.

Steps to solve this

1. Identify all functions that use pspm_write_channel, allow channel_actionto be replace, and do not create channels of new type.
2. Check whether these functions pass correct channel numbers to pspm_write_channels. Channel numbers should correspond to those that were retrieved from the file with a pspm_load_channel function call, or otherwise within the calling function.
3. Compile a list of functions that fulfill 1 and not 2.
4. Implement correct assignment of channel numbers in these functions.