Now checks for missing TOPS or DEPTHZ in DXV etc. initialization.
Corrects required version and make Eigen3 download work.

At least on Debian requiring version 3.1 of Eigen3 is not sufficient
according to issue #25 the minimum version is 3.1.2. This patch fixes
the required version. In addition it skips the REQUIRED option passed
to find_package(Eigen3) to make the git download work.

Fixes #25.
Saturation table scaling and hysteresis.

This commit implements some additional scaling keywords.  This includes
the ISWL-family that provide hysteresis behaviour via alternative
scaling of the tables.  The old parser has been somewhat extended for
testing purposes.  The commit also includes a slight refactoring of the
SatFunc-family where a new base class has been introduced.
RV is back in business.
RV is back in business - a second attempt.
Merge pull request #474 from atgeirr/throw-missing-tops-depthz

Check for missing TOPS or DEPTHZ in DXV/DYV/DZV initialization.
Fix fallout from PR #474

The check introduced in PR #474 (commit ad899fc) means all decks
must supply valid depth data.  Update test decks accordingly to
restore "make test".
Removed blanks from strings.
Merge pull request #483 from bska/fix-test-fallout

Fix fallout from PR #474
Merge pull request #478 from osae/endscale

Saturation table scaling and hysteresis.
Merge pull request #476 from andlaus/fix_eigen3_version

Corrects required version and make Eigen3 download work.