fix 343: add conda_default_allow_channels option

[csadorf]

If set, automatically add new channels to the channels database.

Before merge:

• Implement tests
• Add documentation

Closes #343.

[csadorf]

The new option is set to "True" by default which breaks previous behavior.

[csadorf]

@costrouc I think this is more or less ready, however I think we should at least consider whether maybe instead of adding an additional option, setting the conda_allowed_channels to an empty list to achieve the same behavior would be more intuitive to users.

I also could use some guidance on the test implementation.

[csadorf]

Manually tested with:

channels:
- conda-forge
- bioconda
- defaults
dependencies:
- _libgcc_mutex=0.1
- _openmp_mutex=4.5
- bcftools=1.10.2
- blis=0.7.0
- pip:
  - nothing
- ipykernel
name: map-and-call
prefix: null

[costrouc]

I saw some Black formatting issues and just added two comments on a conditional check that needs to be made. Otherwise this PR looks great to me. Thanks for your work @csadorf

[csadorf]

@costrouc I'd suggest that you consider my suggestion concerning the overloading of the conda_allowed_channels trait, but otherwise this should be ready to merge.

[costrouc]

Thanks @csadorf. Yeah I get what you are saying and that makes sense. I think it would be a good idea to write it like that especially since there is no case where you would want an empty list of channels in the first place. Making it so that we can overload that behavior!

Would you mind making that change? Sorry for the late response