Fix gaussian linear benchmark, expose benchmark simulators.

[vpratz]

The GaussianLinear benchmarks were broken for n_obs not None. I fixed them and added tests for the simulators. I also exposed all simulators from the benchmark_simulators directory in the simulators module.

[codecov]

Codecov Report

Attention: Patch coverage is 77.77778% with 4 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
...simulators/benchmark_simulators/gaussian_linear.py	60.00%	2 Missing ⚠️
...rs/benchmark_simulators/gaussian_linear_uniform.py	60.00%	2 Missing ⚠️

Files with missing lines	Coverage Δ
bayesflow/simulators/__init__.py	100.00% <ø> (ø)
...esflow/simulators/benchmark_simulators/__init__.py	100.00% <100.00%> (ø)
...simulators/benchmark_simulators/gaussian_linear.py	90.47% <60.00%> (+90.47%)	⬆️
...rs/benchmark_simulators/gaussian_linear_uniform.py	90.90% <60.00%> (+90.90%)	⬆️

... and 7 files with indirect coverage changes