Fix multi thread env state in case N=1

[johannes-fischer]

I have come across an issue with MultiThreadEnv for an environment where the state_space(envs[1]= is not a Space. The error message is

ERROR: DimensionMismatch: cannot broadcast array to have fewer non-singleton dimensions

because selectdim(menv.states, N, i) in this case returned a
0-dimensional view(::Vector{Vector{Float64}}, 1) with eltype Vector{Float64}.

I belief this change should fix the problem.

PR Checklist

• Update NEWS.md?
• Unit tests for all structs / functions?
• Integration and correctness tests using a simple env?
• PR Review?
• Add or update documentation?
• Write docstrings for new methods?

[jeremiahpslewis]

Thanks for catching this! Can you add a test which catches this issue?

[johannes-fischer]

Is there an environment where the state space is not a Space? The one I am using is not suitable for a test. Unfortunately, there seem to be no other tests to adapt.

[jeremiahpslewis]

Can you create a minimal custom env extending AbstractEnv?