Catch invalid user-selected solution method

So far, an unsupported user-selected solution method would result in a silent fallback to a supported method.

This could have unintended effects if the user picks a valid method (e.g. fullLAPACK) which is not supported by the current build of pede.

In this case, the code would run successfully, and only a thorough inspection of the logs would identify an undesired solver being used.

Instead, move to throwing an error if the user explicitly specifies a solution method that is not supported by the current build.