This adds a proper MC method and a few example tests for 1D PDFs. This is using Catch2. The old tests have been moved to Catch; workarounds for GoogleTest are no longer needed.

Adds: pdf fillMCDataSimple, pdf fit, and dataset clear; drops old experimental MC function.

I've dropped the requirement that PDFs implement recursive set indices; while that still is overridable, normal PDFs only need to call registerFunction("ptr_to_function", ptr_to_function) (no more macros needed in user code). If they need special setup, populateArrays is overridable (and can be called from the overridden function). Many features, like logging, are now built in and provided by default. See GaussianPDF for an example of how simple and clean PDF definitions can be.

Fit controls are now enum based only; the string method has been removed. getName() on a fit control will produce the old style name.