read_gmx, get_indicies: added optional intent(in) description for user
prompt
main: used new optional index file prompt
read_xtc_file: added line about which file is being read in.
Added new argument passed to read_xtc_file from read_indicies (chosen_group)
Minor edits to stdout for index file prompt
Moved stdout title
Moved output to subroutine in out.f90