RF: Compile `Optimiser`, `LinearSolver`, and `IntensityMapper` classes with `g++` instead of `nvcc`

Paul McCarthyrequested to merge
mnt/compile-optimisers-with-gcc into master

Now that the dependence on CUSP has been removed, the SparseDiagonalMatrixTiled interface is CPU/GPU agnostic, which means that nvcc is not needed to compile the Optimiser, LinearSolver, or IntensityMapper classes.

Edited by Paul McCarthy

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