Full support for Eigen::Map(ped) types

[psiha]

(and Eigen::TensorMap)

AFAICT pybind11 supports Eigen maps ('views') as return values and not as arguments (I get a compilation error about a delete function and in a nearby comment it says that maps are not supported as arguments but without an explanation as to why) and I don't see why this should be the case. In the documentation you recommend the use of the Eigen::Ref template. However Refs are slightly heavier and at the same time less versatile (in certain contexts I care about, like adapting/viewing memory of arbitrary yet 'compatible' objects as Eigen types, e.g. OpenCV Mats) than Maps and my codebase is already based on (that is to say littered with:) Maps so changing all that is not a (happy) option.
Could you add this support?

[bstaletic]

Hi. The problem is that, at this point, I don't think any of the active maintainers knows Eigen. We would need help, in the form of a pull request, preferably.