Add import directive

[meteospike]

Since it is an independent extra feature, it may not be necessary to add a new command line flag. The principal change is probably moving ahead the sequence evaluating the list of modules directories.

Here is an example of the new directive :

program pm_dble
#:import pm, foo
  integer :: i=${int(pm.pm_two+foo.foovar)}$
  write(6,*) 'Begin DBLE'
  write(6,*) 'i =', i
  write(6,*) 'End DBLE'
end program pm_dble

[aradi]

I just went over the PR once more (sorry for the long delay). Are you still interested to work on it? The module path manipulation issue still needs to be fixed (setting back the original path after importing the user specified modules) and tests are missing.