Fix cases where floating point numbers are used as array indices.

[Infinoid]

Python (v3.6.9) fails to execute this code, with errors like the following:

examples$ python3 AFRL_demo.py 
Traceback (most recent call last):
  File "AFRL_demo.py", line 27, in <module>
    [phs, platform] = phsRead.AFRL(directory, pol, start_az, n_az = 3)
  File "../ritsar/phsRead.py", line 76, in AFRL
    R_c = pos[npulses/2]
IndexError: only integers, slices (`:`), ellipsis (`...`), numpy.newaxis (`None`) and integer or boolean arrays are valid indices

This fixes those errors for all of the examples.

[jks7592]

I put the changes you listed that I didn't already have in my fork. Let me know if you see anything else, I've made quite a few changes in trying to get AFRL's Wide Angle SAR to work for me.