swatInitScaling(): Rename array size parameter

This commit renames the 'np' parameter used to allocate small arrays
for saturations and capillary pressures to 'max_np' to better
reflect its purpose.

Suggested by: [at] atgeirr
swatInitScaling(): Name magic constant

The constant 1.0e-8 was used as a threshold to distinguish "low"
from "high" capillary pressure values.  Introduce acutual constant
"pc_low_threshold" to clarify that role.

Suggested by: [at] atgeirr
swatInitScaling(): Remove EOL whitespace

This commit removes several instances of EOL whitespace in function
'swatInitScaling()'.  Aesthetic only.  No functional changes.

Suggested by: [at] atgeirr
swatInitScaling(): Abide by OPM coding conventions

This commit attaches the reference-signifying ampersand to the type
as is preferred in the OPM code base.

Suggested by: [at] atgeirr