General moving-window transformations in boosted-frame simulations

[bnara]

During initialization in boosted-frame simulations, the moving window, of length $L$ given in lab-frame coordinates, is resized along the direction of the boost. Currently, the code assumes that the moving window is traveling along the direction of the boost at the speed-of-light.

This PR generalizes the transformation of the moving window from the lab-frame to the boosted-frame, taking into account the direction and speed of the moving window (c.f., Eq. 1 in Ref. [1]). As an example, let's say the boosted-frame moves with speed $\beta = 0.6$ (gamma_boost = 1.25). If the moving window also travels at speed $\beta$, then its length in the boosted frame is $L \gamma = 1.25 L$ rather than $2 L$, as is currently calculated.

Special cases for negative values of moving_window_v and moving windows not traveling along the boost direction were both checked. In order to have access to the moving window parameters, there needed to be some rearrangement of initialization order.

[1] J.-L. Vay, Phys. Rev. Lett. 98, 130405 (2007)

[bnara]

The diagnostic field output window, specified via diag_lo and diag_hi, could be transformed in the same way:

WarpX/Source/Diagnostics/Diagnostics.cpp

Line 229 in 78ff8bf

if (WarpX::boost_direction[ dim_map[WarpX::moving_window_dir] ] == 1) {