added outer product operator, added outer method to field and linearisation, added integrate method to linearisation,

Related to Issue #62

The reason for this is the following. MultiFields behave very similar to
dictionaries except for the fact that dictionaries are mutable and MultiFields
are immutable. Therefore, MultiField.update() needs to return a new object. That
is different to the dictionary.update() functionality and therefore unintuitive.

MultiField.union() can do the exact same thing as update() used to do.

Similar to dict.update(other_dict)

added an InverseGammaLikelihood which is the right likelihood for fitting Gaussian diagonal covariances