Coerce edge lengths (floats) to reliable strings.

This matches the server's expectation that we're using floating-point
values, effectively reversing Javascript's implicit conversion of (for
example) 3.0 to 3. But to do this reliably, we need to save and transmit
strings instead of floats in our outbound Nexson. Lesser of evils?
Force any string @length values to JS numbers...

This assumes that the new peyotl will recognize "ints" and accept them
as reasonable edge-length values.
Merge pull request #282 from OpenTreeOfLife/coerce-edge-lengths

Coerce edge lengths (floats) to reliable strings.