Add support for reading Fermi/gbm fits file in Eventlist.read

[ankitkhushwaha]

fixes #614

i have changed my approach to read fermi-gbm data.
instead of creating separate function to read fermi-gbm flies, i have made required changes in Eventlist.read to support them as well.

To read gbm files

file used: link

from stingray import EventList
evt = EventList.read('glg_tte_n0_bn210117458_v00.fit', fmt= 'hea')

Reference

def read_gbm_data(filename, emin=None, emax=None):

    hdulist = fits.open(filename)
    header = hdulist[0].header

    elower = hdulist[1].data.field("E_MIN")
    ehigher = hdulist[1].data.field("E_MAX")
    emid = elower + (ehigher - elower) / 2.0

    time = hdulist[2].data.field("TIME")
    channels = hdulist[2].data.field("PHA")

    if emin is None:
        emin = np.min(elower)
    if emax is None:
        emax = np.max(ehigher)
    for c in channels:
        emid[c]

    energy = np.array([emid[c] for c in channels])
    mask = (energy > emin) & (energy < emax)

    time = time[mask]
    energy = energy[mask]
    trigtime = hdulist[0].header["TRIGTIME"]
    hdulist.close()
    evt = EventList(time, energy, header)

    return evt, trigtime

source: @dhuppenkothen

[codecov]

Codecov Report

Attention: Patch coverage is 71.87500% with 9 lines in your changes missing coverage. Please review.

Project coverage is 94.15%. Comparing base (cc4c259) to head (bc55428).

Files with missing lines	Patch %	Lines
stingray/io.py	71.87%	9 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #894      +/-   ##
==========================================
- Coverage   96.03%   94.15%   -1.89%     
==========================================
  Files          48       48              
  Lines        9770     9788      +18     
==========================================
- Hits         9383     9216     -167     
- Misses        387      572     +185     

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[ankitkhushwaha]

i have added the required tests for this pr.
will add more tests in upcoming commits.

i have added a processed version of file. now having size of 80 kb.

@dhuppenkothen , if u can review this it would be beneficial.