Standardize input of dictionary-like options

To try and solve #62 (closed) and also as @isobel.romero-shaw mentioned somewhere else the channel-type list was a bit confusing, this (attempts) to standardize things a little.

1. The two input options channel-dict and sampler-kwargs are specified through a python-dictionary in the ini file.
2. A standardized way of converting the string to a dict is given. This should allow us to then improve it in future (e.g., allow no strings if needed)
3. It removes the use of eval to ast.literal_eval which is safer