add raw activator for quicker runs

[jbae11]

This PR attempts to accelerate depletion runs by:

1. removing superfluous steps associated with OpenMC API (e.g., creating material, reading hdf5 output, checking same values multiples times)
2. Vectorizing time consuming processes (e.g., getting microxs from multigroup flux)

Because this is a less elegant (in terms of code writing) approach, a new file is created instead of overwritting the existing openmc_activator.py

On my MacBook Pro, it took me 127 mintues wall clock time using the previous method, and now it takes ~7 mintues. YMMV, do let me know.

The input/output call is the same, so to test this, just change the first cell in the compare.ipynb notebook:

from openmc_activator import OpenmcActivator, write_markdown_file, read_experimental_data

to:

from openmc_activator_raw import OpenmcActivator, write_markdown_file, read_experimental_data

[yrrepy]

Hello, me again!

Took a browse:

    # no negative numbers please 
    atom_arrs[atom_arrs < 0] = 0.0
    return atom_arrs

I am also encountering negative atom densities, which I want to purge.
I have this which I am just about ready to PR into OpenMC
(https://github.com/yrrepy/openmc/tree/CRAM_clip-neg-atoms)

My solution:
https://github.com/yrrepy/openmc/blob/CRAM_clip-neg-atoms/openmc/deplete/abc.py#L747:~:text=nuclides&text=clip%5Fnegative%5Fdensities&text=clip&text=False%2C-,clip%5Fnegative%5Fdensities&text=self%2Eclip%5Fnegative%5Fdensities&text=clip%5Fnegative%5Fdensities,-self&text=if-,self%2Eclip%5Fnegative%5Fdensities

Demo:
demo.py

Side-bar:
This may also be a convenient location to purge results below X atoms/bn-cm (e.g. ala Origen-MSX default 1e-20). Though currently I do this myself after, in the depletion_results.h5 file and do it in a sweep over all timesteps of a nuclide.

I might attack CRAM sub-steps next to further improve oscillations etc.