Findings cpp_fp_double_backend

Here are some open tasks for further quality enhancement of the new `cpp_double_fp_backend` template

- [ ] #728
- [x] Better infinity catches add/sub. Fixed by 715e0b461e4c4f9771483c5b1bd8b900718779e8.
- [ ] Catch `(max)()` and `lowest()` in `eval_convert_to`-floating-point.
- [ ] Consider `numeric_limits` values particularly `(max)()` and `epsilon()`. Are there even better values for these?
- [x] #727
- [x] Edge cases of `pow(x, a)` function, in other words $x^a$, for $x{\in}{\mathbb{R}}$, $a{\notin}{\mathbb{R}}$. See also #722
- [x] #722
- [x] When testing, it was discovered `cpp_dec_float` does not compile with `complex_adaptor`. Fixed by 1ac05b1832d7c71812ff841d4d1624fe8d511d07
- [ ] The relative errors when testing data from the table with `cpp_double_fp` in `test_pow.cpp` subjectively seem a bit high. Why?
- [ ] Investigate and address the _TBD_ comments in `test_various_edges_more.cpp`

Notes:

- The new `cpp_double_fp_backend` type is composed of two limbs. At the present time, add/sub/mul/div always do full 2-by-2 limb operations. This issue investigates odd double-limb, single-limb operations like the double-fp type being multiplied by a single fp-type having the same width as its constituents.