Add GOES-18 and GOES-19 RSRs

[djhoese]

Code Sample, a minimal, complete, and verifiable piece of code

No code sample, sorry.

Problem description

Trying to process GOES-18 simulated data with Satpy produces an exception in pyspectral when it tries to read the RSR data. The SRFs are available at:

https://ncc.nesdis.noaa.gov/GOESR/ABI.php

FM3 is G18 and I guess FM4 is G19.

Expected Output

Success!

Actual Result, Traceback if applicable

  File "/home/shared/bin/geo2grid_v_1_0_2/lib/python3.7/site-packages/satpy/scene.py", line 843, in _generate_composite
    **self.attrs)
  File "/home/shared/bin/geo2grid_v_1_0_2/lib/python3.7/site-packages/satpy/composites/__init__.py", line 575, in __call__
    red.data)
  File "/home/shared/bin/geo2grid_v_1_0_2/lib/python3.7/site-packages/pyspectral/rayleigh.py", line 248, in get_reflectance
    wvl = self.get_effective_wavelength(bandname)
  File "/home/shared/bin/geo2grid_v_1_0_2/lib/python3.7/site-packages/pyspectral/rayleigh.py", line 187, in get_effective_wavelength
    rsr = RelativeSpectralResponse(self.platform_name, self.sensor)
  File "/home/shared/bin/geo2grid_v_1_0_2/lib/python3.7/site-packages/pyspectral/rsr_reader.py", line 104, in __init__
    "platform name and sensor or filename must be specified")

Versions of Python, package at hand and relevant dependencies

Thank you for reporting an issue !