Help needed: Port rp1_acoustics.f90

This PR contains the changes from #16 and #17. Then it ports `rp1_acoustics.f90`, currently I have just modified the advection example in place (we'll probably create a new directory once we get it working).

Currently, when I run it, it gives me:
```
ondrej@eagle:~/repos/classic/tests/advection(c)$ ./main 
Reading data file: ./setprob.data
         first 5 lines are comments and will be skipped
Reading data file: ./claw.data
         first 5 lines are comments and will be skipped
At line 77 of file /home/ondrej/repos/classic/tests/advection/rp1_acoustics.f90
Fortran runtime error: Index '2' of dimension 1 of array 'ql' above upper bound of 1
```
This is caused by these two lines in `rp1_acoustics`:
```fortran
        delta(1) = ql(1,i) - qr(1,i-1)
        delta(2) = ql(2,i) - qr(2,i-1)
```
Clearly the advection example only had dimension 1, now it has 2.

@mandli, any idea where this gets set? Do you know how to setup the main.f90 to work with acoustics?