Marshmallow removed

This is a minor version increment because Marshmallow has been removed as a dependency.
It was originally added when RCPCHGrowth was part of the server and was used more for API validation than the package.
However, it does leave vulnerabilities in that there is now no error handling when the wrong type is passed to a function.

This minor version also updates fictional child so as not to cause errors for measurements outside the thresholds of the data.