Bugfix/set default values of some inputs

• Set defaults for fphi(=1.0), fapar(=0.0)

• Ensure alpmhd and kp have default values given

• Ensure fields are initialised in nonlinear_terms unit test

• Remove/simplify diagnostic settings in gs2_init unit test

• Add a comment to warn about a known potential use of uninitialised data

• Provide initial value for surfarea to avoid possible use of uninitialised variable

• Ensure rowblock data is initialised to 0.

  This is important when using tuneminnrow to avoid potential uses of uninitialised data

These are a collection of fixes that were discovered by compiling with all floating data initialised to a signalling Nan and then running the unit tests.