Allow user to select frequency units in RRFMuon

[raquelalvarezbanos]

Fixes trac issue #11028

To test:

• Check code changes (.cpp, .h, unit test and documentation)
• Try user example and play with the different options

[raquelalvarezbanos]

Jenkins, retest this please

[eXeC64]

Looks good, there's just a couple of things I don't understand.

• Reference 1 The TS_ASSERT_DELTA makes sense, but if we want to be sure they're the same value, ignoring precision errors, why have TS_ASSERT_DIFFERS? Wouldn't them having the same exact value just be a happy coincidence, not an error?
• Reference 2 This may be my ignorance showing (I'm just a lowly computer scientist after all 😉), but a comment explaining what exactly the 135.538.. value is and where it comes from would probably be useful for future reference.

[raquelalvarezbanos]

You are right about Reference 2, I should have added a comment. That number corresponds to the muon gyromagnetic ratio (in MHz/T), I will update the code accordingly.
Regarding Reference 1, what I want to check is that both results are similar, but they should not be exactly the same. If they were, something would be wrong. I will also add some explanation.
Thanks for your comments :)