Fix named parameter overload of boykov_kolmogorov_max_flow

[sebrockm]

Fixes #232

[jeremy-murphy]

So I take it that test of the bundled properties failed until you fixed the return type of the named parameter overload?
I just want to understand exactly what the change fixes. And why does it need to use put instead of operator[] now?

[jeremy-murphy]

And... thank you!

[sebrockm]

@jeremy-murphy Yes, the return type was the only error. It always expected the value type of the interior capacity property map, never the one provided via named parameters. (I sometimes hear people saying that they don't like auto as return type. From now on I will point them to this error 😅 )
put instead of operator[] was honestly just a leftover from some early trials of investigation of this error. At some point I had a test that used a property map that didn't have operator[] and compilation failed. The test itself turned out to be non-constructive for identifying this issue, so I removed it. But I kept the put because I think writable property maps are not required to support operator[], only put.