support astropy 2 and 4

[sbailey]

This PR makes a small update to support astropy 4.x, while still retaining compatibility with astropy 2.x. The current version of simqso + astropy 4.x crashes upon import:

In [1]: import simqso                                                           
...
/global/common/software/desi/users/sjbailey/simqso/simqso/hiforest.py in <listcomp>(.0)
     27     linelist = fits.getdata(os.path.join(datadir,'all_lin.fits'))
     28     Hlines = np.array([i for i in range(linelist.size) 
---> 29                            if b'HI' in linelist.ION[i]])
     30     transitionParams = {}
     31     for n,idx in enumerate(Hlines[::-1],start=2):

TypeError: 'in <string>' requires string as left operand, not bytes

@imcgreer after reviewing and merging this PR, could you please make a new simqso tag so that we can use it in our next DESI software release? Thanks.

Background

In astropy 2.x, linelist.ION is a numpy.ndarray object and linelist.ION[0] is numpy.bytes_ thus requiring code like the current if b'HI' in linelist.ION[i]. But inconsistently, linelist['ION'] is a numpy.chararray and linelist['ION'][0] is a str requiring comparisons like if 'HI' in linelist.ION[i] instead (note 'HI' str not b'HI' bytes).

astropy 4.x (or maybe 3.x) made these consistent with both linelist.ION and linelist['ION'] being numpy.chararray objects with individual elements that are str, thus breaking the previous comparison with b'HI'. The syntax that works with both astropy 2.x and 4.x is:

if 'HI' in linelist['ION'][i]]

[imcgreer]

Thanks for the detailed explanation!

[sbailey]

Thanks @imcgreer