[Feature Request] Run to time limit for Neal and Greedy

Would be useful to have a `time_limit` argument for Neal and Greedy.
Both Neal (for a fixed `num_sweeps`) and Greedy can implement a run-to-time-limit feature by adding timing calls per read, i.e., variable number of reads depending on time constraint. Example from [RandomSampler](https://github.com/dwavesystems/dwave-samplers/pull/24/files#diff-04d125600776f967690134c316314f47d56d0e0add77eb252fcd60c1fb52887cR130).