add a function to add operational period

[caver456]

Just seems like this is needed for completeness for sartopo_python 2.0.0 - after adding this function, there should be enough functionality to create all of the key aspects of a search just from code, if that were ever a need for some reason.

And - it should be pretty easy to implement.

[caver456]

Note that request URLs are case sensitive: /Operationalperiod fails but /OperationalPeriod works. Add some code in apiUrlEnd processing inside sendRequest to account for this case.