Save an evaluated inputs file on start

[titoiride]

Hi all,
talking with @ax3l and @n01r we thought it would be cool and useful to have WarpX dump an "evaluated" version of the used inputs on start, particularly when using text files as input.
It currently returns the "used input" file, but that only lists the input as they were requested in the file.
If some of the parameters are expressed using constants or mathematical expressions, the file returns the expression as it is.
Perhaps in addition to the file already dumped it would be useful to have one where constants and mathematical expressions are return evaluated so that it's possible to catch bugs related to their definition.

[ax3l]

@WeiqunZhang, do you think we could create a file similar to warpx_used_inputs that has amrex::ParmParse constants evaluated in it? Something like warpx_evaluated_inputs.

[ax3l]

Example:

WarpX/Examples/Tests/Implicit/inputs_1d

Lines 1 to 24 in ce7f5cc

	#################################
	############ CONSTANTS #############
	#################################
	my_constants.n0 = 1.e30 # plasma densirty, m^-3
	my_constants.nz = 40 # number of grid cells
	my_constants.Ti = 100. # ion temperature, eV
	my_constants.Te = 100. # electron temperature, eV
	my_constants.wpe = q_e*sqrt(n0/(m_e*epsilon0)) # electron plasma frequency, radians/s
	my_constants.de0 = clight/wpe # skin depth, m
	my_constants.nppcz = 100 # number of particles/cell in z
	my_constants.dt = 0.1/wpe # time step size, s
	#################################
	####### GENERAL PARAMETERS ######
	#################################
	max_step = 100
	amr.n_cell = nz
	amr.max_level = 0
	geometry.dims = 1
	geometry.prob_lo = 0.0
	geometry.prob_hi = 10.*de0

Regarding replaced for amr.n_cell and geometry.prob_hi.

[ax3l]

@WeiqunZhang mentioned that currently we will see in warpx_used_inputs:

amr.n_cell = nz
amr.n_cell = 40
geometry.prob_hi = 10.*de0 
geometry.prob_hi = <single-number>

(related to AMReX-Codes/amrex#4087 ). We recently updated the sorting, before that the lines were a bit more apart from each other, this was pulled into development last week: AMReX-Codes/amrex#4101