Some refactoring

[rboston628]

Some major refactoring added to code.

• All root-finding methods moved to library, rather than re-implemented everytime they are used
• mode_finder will automatically find the $k=0$ mode, for calculating overlap, without needing to repeat this in the function to calculate overlap: it can be assumed $k=0$ (or $k=1$) exists.
• The mode_finder routine now uses stdlib containers set and unordered_map to more easily track which modes have been found and which need to be found while running calculation
• Consequently, now requires C++14 or later to compile
• CHWD++ models implemented in Newtonian and 1PN physics
• Some compatibility errors from missing -lm flag fixed by changing compiler flags. For reasons unknown, my particular compiler on my particular macbook automatically links in the math library, which is not universal behavior.
• There were minor errors in the boundary conditions of the 1PN nonradial mode calculation, which have been fixed.

All behavior should be consistent with prior version.

[rboston628]

Annoying issue of warning messages from the matrix inversion method.

Either need to implement failsafe in the matrix inversion method to stop NaNs from happening at all, or find a better way of handling what happens if they do occur.