Convert BigDecimal to Float prior to JSON encoding

Rails 4.0 changed the default behaviour to encode BigDecimals as strings
(which our apps do not expect). 4.1 removed the ability to encode as
floats altogether.

The scenario presenter will now check for BigDecimals, and convert them
to floats prior to encoding.

Ref quintel/etmodel#1831
Save all FCE attributes when calculating

We were discarding the individual extraction, treatment, transportation,
and conversion attributes for FCE-enabled carriers when FCE was disabled
in the scenario, and just keeping the combined value.

The FCE calculator has been adjusted to retain these values, so that
they may be displayed in the FCE chart.

Closes quintel/etmodel#1831
Fix FCE spec. :/