add "sections" item to custom "RHO" keyword

this is required after OPM/opm-parser#296 has been merged, but it can
be merged before...
Merge pull request #131 from andlaus/add_sections_to_RHO

add "sections" item to custom "RHO" keyword
Upscaling utilities: Use raw property vectors where appropriate

We need access to the raw, unconverted property vectors to use the
existing algorithm for defining (e.g.,) capillary pressure ranges.
This subtle point was missed during the initial transition to using
the opm-parser module for input handling.
Upscaling utilities: Don't hard-code numeric conversion factor

This commit replaces the hard-coded conversion factor from mD to
m**2 (metres squared) with an expression based on the Units.hpp
facility of module opm-core.

Note: This may introduce slight changes to the upscaling result due
to reduced rounding error in the unit conversions.