Gridpp

* API changes:
- Support for cartesian x/y coordinates (instead of just lat/lon)
- A bilinear interpolator that works on all grid rotations
- Easy cross-validation setup for optimal interpolation
- Functions for computing distance to nearest points on grid
- Functions for computing number of nearby points on grid
- Function for computing neighbourhood quantiles with spatially varying quantiles
- Function for gridding observations
- Function for filling in missing values
- Experimental method for optimizing a metric (metric_optimizer_curve)

* General changes:
- Changed the licence from GPL-2 to LGPL-3
- Exposed an API to a library of core gridpp functionality
* Command-line changes:
- Added window= to -c deaccumulate

* New features:
Variables are referenced by their NetCDF variable name
- Adds cmake compilation
- Adds -c oi to perform optimal interpolation
- Gradient downscaler can use temperature at model levels (instead of T2m)
when computing gradient
- Adds coastal regression in -d gradient
- Adds -c coastal, a bias-correction scheme that weights land and sea
tempatures from a neighbourhood
- Variable names can be overridden by passing in a variable map file
* Backwards-incompatible changes:
- Gradient downscaler options are different. Use -d gradientOld to get
previous behaviour.
- Parameter text files must contain header
- Parameter files are specified differently: -p <filename> type=<type>
- The command-line interface to this version has changed significantly.
- Names as they appear in the file are used to identify variables. I.e.
use -v air_temperture_2m instead of -v T.
- -c diagnose replaced by -c diagnoseHumidity and -c diagnoseWind

* Adds -d bilinear
* Adds bilinear interpolation to -d gradient

* Adds -c diagnose for dewpoint temperature and RH
* Diagnoses altitude from geopotential height for EC files
* Fixes bug in Kalman Filter formulation

* Fixes bug with -c diagnose in multi-thread mode