MockDWaveSampler initial_state format is incorrect

**Description**
MockDWaveSampler handles initial_states differently to DWaveSampler, formats are incompatible.
This issue is complicated by suboptimal documentation https://docs.dwavequantum.com/en/latest/quantum_research/solver_parameters.html#parameter-qpu-initial-state, and related issues
https://github.com/dwavesystems/dwave-cloud-client/issues/700
https://github.com/dwavesystems/dwave-system/issues/299

**To Reproduce**
Passing initial_states as a dictionary or list of integers works for DWaveSampler, passing a list of tuples works for MockSampler, no compatible format exists. MockDWaveSampler can be corrected as other issues are addressed.

**Expected behavior**
MockDWaveSampler should accept the same format(s) for initial_state as DWaveSampler, to enable testing.

**Environment:**
- OS: [Ubuntu 16.04.4 LTS]
- Python version: [e.g. 3.7.0]

**Additional context**
Add any other context about the problem here.