Rename double precision constant r_2 (CABLE-POP)

[mcuntz]

I propose to rename the constant r_2 to r2.

The undescrore is a bit annoying and makes constants rather lengthy because there are two undescrores then, e.g. 1.0_r_2.
I would have even proposed to remove the number in the name because I find 1._r_2 a bit confusing. 1._r2 is already a bit better.
Some people, like me, use dp for double precision, which gives 1._dp or 1.0_dp.

I also propose to remove underscores in r_1 and i_d as well. I just realised that I call i_d normally i4, which has also a number at the end :-( But I do not use it much anyway (but I use i8).