Add a subsection for modification of variable bounds in docs

[fwitte]

I was trying to modify variable bounds instead of creating additional constraints for already instantiated variables and could not find a subsection in the respective chapter. It does not work with something like variable.loc[slice, other_slice].lower = 42 and I did not immediately understand why, so I thought a hint on how to do this might be useful.

If this is bad practice, I would be happy to get a rejection of the PR (and also be curious why it would be :)).

Thank you!

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[fwitte]

I blindly overlook the section, I was only reading the variables one... Nevertheless, including how to slice variable.lower.loc[slice, other] instead of variable.loc[slice, other].lower might be useful

[FabianHofmann]

@fwitte thank you for your contribution. Setting the bounds on a selection of variables does not work, as the .loc function returns a copy of the original data. Therefore, the bound will only set to the copy and not the variable selection itself. Would you mind adding a note there, that this case is not supported (yet)?

[FabianHofmann]

I blindly overlook the section, I was only reading the variables one... Nevertheless, including how to slice variable.lower.loc[slice, other] instead of variable.loc[slice, other].lower might be useful

no worries

[fwitte]

It does work, if you access the xarray.DataArray like this: variable.lower.loc[slice, other], not sure if it is intentional.

[fwitte]

I added the hint and explanation, which of both version work. It is now in the Variables section. If this is of interest, let me know if I should reopen the PR and/or move the information to the Modifying the Model section.

Have a nice day