Re: [eigen] On a flexible API for submatrices, slicing, indexing, masking, etc.

[Yuanchen Zhu <yuanchen.zhu@xxxxxxxxx>]

- The word "range" is overloaded much more than span. There's boost::range, the WIP range v3, and the "range-based" for loops. These big shots can have different conventions, some times [first, last) (range-based for), some times [begin, end) (boost's irange). For span, the only conflict is gsl::span right now (which should really be named array_view anyway, akin to std::string_view. It also takes in (base_ptr, length), so it's categorically different from our use).

​I made a typo here: I meant boost's irange uses [first, last], and range-based for loops uses [begin, end).​