Missing include of `optional`

[adv4ncr]

Hi Sven,
i think you missed #include <optional> in the header:

EiCOS/include/eicos.hpp

Line 3 in 87398a9

#include <Eigen/Sparse>

[adv4ncr]

And a semicolon here in the README:
https://github.com/EmbersArc/EiCOS/blob/87398a9c4732b09c5b2524470b67ad583719423b/README.md?plain=1#L78C1-L78C15

[EmbersArc]

Hey! Indeed, newer compilers require explicit includes. Feel free to open a PR :)