Repeated boilerplate across Inference classes: derive from common parent

[alvorithm]

Problem

Some of the argument checking across SnpeBase, SNL and SRE is repeated code.

Solution

Refactor that commonality and others into a common parent class, NeuralInference.

Follow-up

Each of SNPE A, B, C only override/define one method: _get_log_prob_proposal_posterior(self, inputs, context, masks); consider dealing with it in SnpeBase directly - unless we dispatch elsewhere on concrete class.