Reading parameters for xpxspec from csv file

[albertomanfreda]

Currently there is no way to specify a range for the parameters, only the initial value. Reading from file allows the user to pass all the arguments allowed by xspec for a model parameter (value, delta, min, bottom, top, max).

[albertomanfreda]

Tentative implementation in #627

[albertomanfreda]

Now that I am at it, I realized that it may be even better to read the entire model from file, with a structure similar to the one in ixpeobssim/ixpeobssim/srcmodel/ascii/powerlaw_parfile.par

This could be given directly through the --model option, although that requires implementing some logic to understand whether the argument is a file path or a model expression.