Bug: Encoder kwargs strangely sensitive to format

# Issue
I have noticed that if one supplies
```julia
emulator = Emulator(mlt, train_pairs; encoder_schedule = encoder_schedule, encoder_kwargs = (; obs_noise_cov = Γ))
```
Then one has success, while
```julia
emulator_kwargs = (; obs_noise_cov = Γ)
emulator = Emulator(mlt, train_pairs; encoder_schedule = encoder_schedule, encoder_kwargs = emulator_kwargs)
```
gives an error. of the form `Unknown keyword argument "obs_noise_cov" in initialize_and_encode_with_schedule!(...)`

We should ensure greater robustness when providing such kwargs.