Combine duplicates of sort_array_coordinates.

This unfortunately causes a bit of extra allocation for unused
parameters. Hopefully I can find a workaraound for that.
Replace Numerical Routines sorting code.

Numerical Routines source code is not re-distributable in any way at all
and is 100% incompatible with the GPL.

The replacement code originates from the heap_sort.f90 in the mesher
code. It appears to be a fairly straight-forward translation of the heap
sort algorithm (e.g., from RosettaCode) and should be unencumbered. I
have added additional parameters in order to sort multiple arrays at the
same time. Interestingly, this code is slightly faster than the
previous version.
Remove get_value_string subroutine.

All it does is copy the default value to the result.
Normalize case of some Fortran keywords.

There are a couple of files with code I'd rather not touch right now.
Remove err_occurred function and commented code.

All the parameter reading subroutines report errors through an argument,
so this function does nothing. The variable it returns is never assigned
the errors anyway.
Remove unnecessary string trimming.

These strings are trimmed when extracted from the Par_file, so there's
no need to do it again. What's worse in this case is the string is
trimmed in a loop over all receivers.
Remove Par_file variable prefixes.

These are not used by the Par_file parsing code. Some places even didn't
have them.
Small spelling fixes in comments.