Intel Fortran support

[MaximeVdB]

This PR brings support for the ifort Intel Fortran compiler (in part because LIS seems to have better support for it, compared to GNU Fortran).

You can use ifort by specifying FC=ifort in your make command (otherwise the FC=gfortran default is used).

Porting to Intel Fortran mainly involved changes to global variables of allocatable string type, requiring a different implementation of the corresponding getters and an explicit initialization (to empty strings).

As such, this yielded incorrect results in the Europe tests with both gfortran and ifort, unless the -fPIC flag got omitted (as e.g. happens when compiling with DEBUG=0).

Digging deeper revealed that this was due to the following combination of events:

• The AdvanceOneTimeStep reorganization introduced a bug causing the WPi variable to be uninitialized upon entry of AdvanceOneTimeStep (while it should have been passed on from a previous time step).
• Previously, this bug seemed to have no real effect, presumably because the same memory/cache location is being used for the WPi variable and because this location happened to not get overwritten.
• Using the different getter implementation (with allocatable strings instead of fixed-length strings) now breaks this behavior and exposes the bug (presumably by overwriting the WPi memory address at some point).

By propagating the WPi value from one time step to the next (commit a05d037695c9ebb8c3897f23513b414881e9543d), the full testsuite passes in all cases:

• [x] DEBUG=1 FORTRAN_EXE=1 and DEBUG=0 FORTRAN_EXE=0 for FC=gfortran with both the foss-2018a toolchain and the Singularity image.
• [x] DEBUG=1 and DEBUG=0 (both with FORTRAN_EXE=1) for FC=ifort with the intel-2018a toolchain.

[mbechtold]

Successful runs with LIS - AC70 compiled with ifort.