Add detrend function

[mmccrackan]

This branch adds a detrend function that duplicates the functionality in sotodlib's detrend_tod function for 1D or 2D arrays (sotodlib supports higher dimensions, but I haven't reproduced that here for now). It has mean, median (using GSL median which is much faster than a std::sort approach), and linear detrending along the rows of an array.

This is primarily intended to be used in an upcoming Welch PSD function but is also standalone, so the underlying function can be run sequentially and called within a parallel loop inside another function (more efficient for the Welch PSD calculation).