Added support for missing value in MCMC samples.

Refactoring of the Chain type to allow for missing values (#18)

* adding union of Missing type, removed StatPlots

* work in progress

* changed in favour of #16, added unique names field

* cummean now works for missing values

* modified plotting functions to work with missing values

* added some tests for missing values, fixes for missing value

* added further tests, assertions and minor changes

* all test are now passing