Improve installation time via shallow updates of submodules

[carrascomj]

Please include a short description of enhancement here During installation, submodule updates were taking hours. Shallow updating made it much faster for me (under a minute).

Just a suggestion since there are some trade-offs:

• I don't know if this syntax works on non-POSIX shells.
• It may vary with between git versions; although it should not be a problem since it is just about setting an env variable.

Also, setting a jobs=$n_cores env variable might also shorten the installation time.

I hereby confirm that I have:

• [ ] Tested my code on my own machine
• [ ] Followed the guidelines in the Contributing Guide
• [ ] Selected develop as a target branch (top left drop-down menu)

(Note: You may replace [ ] with [X] to check the box)