Updating write functions to take separator arg.

Updating all array write functions in `array_utils` to require
a string separator argument.
Updating array write function calls in `dpdraw`.

Updating array write functions to provide string separator argument.
Updating array write function calls in `intpart`.

Updating array write functions to provide string separator argument.
Updating `eureject`.

Updating `eureject` to keep summaries of all files/samples used
for calculating means/std devs and for rejection. Also, adding
`write_summary` function to report all of this information to
stderr.

Also, updating all array write functions to provide separator
argument.

Updating tests accordingly.