Bug in f_salpeter and f_chabrier, NEW: test_req...

- Fixed bug in place where dN is set to 0 outside of
  the input mass limits.
- The previous expression was the following:
  dN_arr[(mass_arr < mass_min) & (mass_arr > mass_max)] = 0.0
- It fails because being an AND, no element satisfies the condition.

- Added an option for normalize=False so that can compare
  two different LF integrals with different mass bounds in
  estimate_required_n.

- Updated in both myanalysis.py and test_simulate_cmd.py

- Implement test_required.py within myanalysis.py to test
  new module estimate_required_n(...). While running this
  program found error in f_salpeter