Refactor interfaces/txt.py use of chtrig channel

[smoia]

After the discussion in #272 , I understand how the current use of chtrig in interfaces/txt.py can lead to confusion.
I'd propose a very quick, very neat refactoring to lower the chances of confusion while reading the code

Detailed Description

In this line and this line, interfaces/txt.py initialise a BlueprintInput object with chtrig +1 as trigger channel.
This could be thought coming from the idea that the first channel becomes time, however it's due to the fact that upstream in the process, here, chtrig is decremented by 1. This could lead to confusion in understanding the script, especially since chtrig is not used in any other place than the BlueprintInput initialisation.

Possible Implementation

• Delete line 322
• Remove chtrig from this definition's argoments, since it's not used in the definition itself.
• Change chtrig +1 in this line and in this line into chtrig only.
• Add explanation of chtrig indexing starting from 1 for human readability in all the docstrings.

I'm going to add a Good First Issue label since it should be not that hard. @RayStick you're still the guardian of G1Fs.

[vinferrer]

Did we stablish a limit on how much time a good first issue can be unresolved?

[vinferrer]

Since This issue is affecting the implementation of #257 we should move to implement this. If someone with less experience in scripting the toolbox wants to try good. If not I am happy to do this quickly

[eurunuela]

I'd say you could start to work on it, it's already been two months.