Added logarithmic units

[Atmelfan]

Poor attempt at implementing #201.

No tests, doesn't compile under no_std, missing documentation, fails CI, etc. Breaks every rule regarding macros and generic programming.

[iliekturtles]

Thanks for the PR! My plan right now is to finish up the two other open PRs thenpush out a new version. After that I'll dig into this PR in more detail.

[iliekturtles]

I finally released v0.31.0 earlier today and will start reviewing this now. Took me a bit longer than expected!

[iliekturtles]

Could you possibly rebase this on top of master? The recent change to extract some of the code from quantity.rs into unit.rs is causing conflicts with this PR. I started looking to do this but never ended up finishing with the limited time I've had recently.

[Atmelfan]

I'll take a look over the weekend.

[iliekturtles]

Finally got a chance to rebase. https://github.com/iliekturtles/uom/compare/Atmelfan-master Will start reviewing for real now!

[iliekturtles]

I finally finished my library book, which I'm going to blame any delays on. I have been reading about power and root-power quantities and digging in to this more and wanted to provide an update. The trait changes mostly need minor fixes like documentation and formating. I believe the to/from_linear functions should be merged into to/from/change_base. I'm reviewing rustc/llvm optimizations next to ensure that doing so will keep these functions zero-cost.

[iliekturtles]

@Atmelfan, I'm starting to look at integrating these changes again after finally merging #260 and was wondering why the Ratio's decibel is defined as a root-power ratio (10^(1/10)) but Power's decibel-watt derivatives use power ratios (10^(1/20)).

[Atmelfan]

It seems right to me, but I might be reading it wrong?

Ratio, base 10, scale 20:

        @part_per_billion: 1.0_E-9; "ppb", "part per billion", "parts per billion";
        @part_per_trillion: 1.0_E-12; "ppt", "part per trillion", "parts per trillion";
        @part_per_quadrillion: 1.0_E-15; "ppq", "part per quadrillion", "parts per quadrillion";
+
+        @decibel: 1.0, 10.0, 20.0; "dB", "decibel", "decibels";
    }
}

Power, base 10, scale 10

+        @decibel_watt: prefix!(none), 10.0, 10.0; "dBW", "decibel-watt", "decibel-watts";
+        @decibel_milliwatt: prefix!(milli), 10.0, 10.0; "dBm", "decibel-milliwatt", "decibel-milliwatts";// dBm is more common than dBmW
+        @decibel_microwatt: prefix!(micro), 10.0, 10.0; "dBµW", "decibel-microwatt", "decibel-microwatts";

[iliekturtles]

Logarithmic units require pow/root functions which are not available
in no-std/no-libm builds. Excluding logarthmic units has proven
difficult (in-place filter foiled by macros unable to be used inside an
enum definition). Using a tt-muncher could possibly solve the issue, but
would likely hit the recursion limit very quickly and likely the
quadratic performance characteristics would make uom's slow
compilation even slower.