Chain rule for nonlinear compositions

What's Changed

• adds fast blas copy for sparse vectors by @Transurgeon in #51
• [Ready for review] Add vstack atom through transpose(hstack(transpose[args])) by @Transurgeon in #52
• Numerical derivative checker by @dance858 in #56
• split up elementwise atoms by @dance858 in #57
• [WIP] jacobian chain rule by @dance858 in #58
• chain rule wsumm_hess by @dance858 in #59
• add work struct for expression by @dance858 in #60
• copy sparsity function by @dance858 in #61
• [WIP] more tests by @dance858 in #62
• Folder structure refactor by @dance858 in #63
• [WIP] chain rule for jacobian in multiply by @dance858 in #64
• [WIP] Chain rule for hessian of multiply by @dance858 in #65
• [Ready for review] Quad form chain rule by @dance858 in #66
• [WIP] Matmul chain rule by @dance858 in #67
• move old code into old-code-folder by @dance858 in #68

Full Changelog: v0.1.5...v0.1.6