Avoid leading underscores in identifiers

Summary

Identifiers with leading underscores are reserved for the implementation

https://timsong-cpp.github.io/cppwp/n3337/reserved.names#global.names-1.2

We are using leading underscores at several places, which should be avoided.

Environment

• Operating System : all
• Architecture : all
• Eigen Version : 3.3.x/master
• Compiler Version : N/A
• Compile Flags : N/A
• Vector Extension : N/A

Minimal Example

It's not hard to find individual examples were we use this. I don't know if there are tools which warn against user-defined identifiers with leading underscores.

What is the current bug behavior?

No actual problems so far, but small risk of breaking code with future compilers

What is the expected correct behavior?

Don't use reserved identifier names.