Raise statements for incompatible inputs into FittingSequence

[sibirrer]

Many inputs into the FittingSequence() class are demanded to be rather specific, with errors caused down-stream without much indication of why and where the input is inconsistent. For example:

• the initial guess spreads 'sigma' values should be > 0, otherwise the PSO or MCMC algorithm breaks down.

Having better raise errors might help the user adjust the settings accordingly.