Modify "rkf45.f"

Add interface for derivative functions.
Create "d1mach.f"

Called by "rkf45.f".
Create "rkf45_test.f90"

A testing program for "rkf45" solver.
Create "machine.f"

A new version of "d1mach.f".
The old version gives an error when 16-bit double precision is used,
as compiled by "gfortran -fdefault-real-8".
Add makefile for "rkf45_test.f90"
Modify ".gitignore"

Ignore files produced by compilation.