Merge branch

UPDATED LIBRARY:
    - added DERIVATE module. containes routines to evaluate jacobians, gradients and
    derivative of functions, D and C, subroutine and function form.
    - moved grids generation into a separate module ARRAYS.
    - moved error and convergence evaluation into ERROR module.
    - rename SLPLOT/SLREAD to IOPLOT/IOREAD and associated headers file, so to have a
    better naming convention. Updated to F21.12 (was F18.10).
    - VECTOR algebra module is now entirely independent.
    - fix bug in STATISTICS when evaluating histograms
    - avoid using linspace in FFTGF. NOTE that FFTGF needs a serious update.
    Possibly avoiding default use of MKL library. It would be better to
    rely of FFTW3 or FFTPACK. Some tests are in order.
    - TOOLS added function to evaluate order of magnitude of a real
    - ERROR added reset flag to avoid resetting the error check
Conflicts:
	src/TOOLS.f90
Merge branch 'devel'