Add support for negative rates on barrier options.

[mwmiller]

Instead of flooring to zero as done in the pricing module, here we need
a very small value to avoid similar divide by 0 problems. For this,
we use the "SMALL_VALUE_MU" which seems tailor-made for the purpose.

Also, some cleanup and rationalization of the test file to make it
easier to understand both the source and output. Plus, of course, adding
a couple test cases for the improved functions.

All of the values seem reasonable to the untrained eye.

cc: @junbon @rakshit-binary @kavehmz @jy-binary

NB: the travis tests may fail if Binaries v1.22 is not yet widely available. This will not be merged until tests pass, but no further code changes are expected to make that happen.

Edit to add: tests pass, so I guess you can ignore the above.

[coveralls]

Changes Unknown when pulling 9e034e6 on negative_rates into * on master*.

[mwmiller]

Anybody have any feedback on this? It's a set of nearly identical changes as those to the pricing formulae.

[rakshit-deriv]

So essentially for greeks, since we can't floor them to zero, we are limiting these to a SMALL_VALUE_MU, right ? It looks fine to me.

[mwmiller]

since we can't floor them to zero, we are limiting these to a SMALL_VALUE_MU

Correct. Since it's used in multiple places in both distributions (and the numbers came out "right"), I decided to stick with convention. For the record, the value is (currently?) set to 1e-10.

I'll get this out and into the system today if possible.