Updated environment variable handling

ISMdust XSPEC models now read the ISMDUSTROOT environment variable using Fortran 90 "getenv" command instead of "fgmstr". Tested and confirmed on Ubuntu 16, Heasoft 6.25

ISMdust cross-sections computed with higher resolution

Re-computed the ISMdust cross-sections with higher resolution in the C K and Fe K edges.

This was done so that ISMdust cross-sections may be compared with the Fe K absorption component for olivine, provided by Rogantini et al. (2018).

ISMdust with additional olivine absorption model

A library of extinction, absorption, and scattering cross-sections for typical Milky Way dust using the optical constants of Draine (2003). An additional absorption-only model provides high resolution Fe K absorption features from olivine (Rogantini et al., 2018).

ISMdust model from Corrales et al. (2016)

This release bookmarks the version of ISMdust released with Corrales et al. (2016) -- http://adsabs.harvard.edu/abs/2016MNRAS.458.1345C