Update CHANGELOG.md

CHANGELOG.md

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 ### New features
 
+* [Issue 33](https://github.com/MassimoCimmino/pygfunction/issues/33) - New class `gFunction` for the calculation of g-functions. The new class is a common interface to all boundary conditions and calculation methods. The new implementation of the solver reduces the memory requirements of pygfunction. The new class implements visualization features for the g-function and for heat extraction rates and borehole wall temperatures (both as a function of time and for the profiles along the borehole lengths).
 * [Issue 75](https://github.com/MassimoCimmino/pygfunction/issues/75) - New module `media` with properties of brine mixtures.
 * [Issue 81](https://github.com/MassimoCimmino/pygfunction/issues/81) - Added functions to find a remove duplicate boreholes.