Decoder solvers now output helpful info.

- They all record the RMSE of the solution
- Some record other information about the solve as well
Minor improvements to decoder solvers.

- Fixed problems with _block_conjgrad with vector sigma input.
- Moved import statements inside solvers.
- Allowed sub-solver to be specified.
Fixed spike rate test

- was wrongly filtering spikes after a previous PR to remove
  cycles from the compute graph.
Connection functions can return 0-d and 1-d arrays
Gave subsolver test looser tolerances

- it was breaking on some machines