Standard MIDAS regressions

[jstriaukas]

Translating standard MIDAS regressions (optimization based on analytical gradients) from R:

https://github.com/jstriaukas/midasml/blob/master/R/midas.solvers.R

function: optim_ardl_beta

[slim-patchy]

Let me start work on this issue soon.

[slim-patchy]

Just added a draft for the function optim_ardl_beta in https://github.com/jstriaukas/midasmlpy/blob/master/midasmlpy/midas_solvers.py.