deprecate RecursiveVec, improve x0 initialisation for GPU #176

Job	Run time
Julia 1.6 - ubuntu-latest	2m 13s
Julia 1.6 - macOS-latest	2m 18s
Julia 1.6 - windows-latest	2m 11s
Julia lts - ubuntu-latest	2m 12s
Julia lts - macOS-latest	2m 13s
Julia lts - windows-latest	2m 9s
Julia 1 - ubuntu-latest	2m 11s
Julia 1 - macOS-latest	2m 16s
Julia 1 - windows-latest	2m 12s
Julia ${{ matrix.version }} multithreaded - ${{ matrix.os }}	1s
	19m 56s

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