Add global pROM example for grad-div problem

[ckendrick]

This adds a global pROM example for the grad-div problem based on MFEM example 4.

Simple example:

./grad_div_global_rom -offline -f 1.0 -id 0
./grad_div_global_rom -offline -f 1.1 -id 1
./grad_div_global_rom -offline -f 1.2 -id 2
./grad_div_global_rom -merge -ns 3
./grad_div_global_rom -fom -f 1.15
./grad_div_global_rom -online -f 1.15

Larger example:

./grad_div_global_rom -offline -f 1.0 -id 0
./grad_div_global_rom -offline -f 1.05 -id 1
./grad_div_global_rom -offline -f 1.1 -id 2
./grad_div_global_rom -offline -f 1.2 -id 3
./grad_div_global_rom -offline -f 1.25 -id 4
./grad_div_global_rom -offline -f 1.30 -id 5
./grad_div_global_rom -merge -ns 6
./grad_div_global_rom -fom -f 1.15 -visit
./grad_div_global_rom -online -f 1.15 -visit

Sample output of larger example (ROM):

#   || F_h - F ||_{L^2} = 0.0384618
#   Relative error of ROM solution = 1.72027e-07
#   Elapsed time for assembling ROM: 6.465071e-02 second
#   Elapsed time for solving ROM: 9.590000e-07 second

[chldkdtn]

Looks great!