add SPDX-License-Identifier and fix copyrights (part 1)
@rcurtin Most of the files in include/bandicoot_bits have been processed, excluding some fn_*.hpp, as well as the opencl and cuda directories. Need to get to sleep now.
Please merge this when possible, otherwise I'll merge this tomorrow. I'll open a second MR tomorrow to finish up the leftover files.
I've added your name to files which have been clearly modified/extended by you. Stuff which looked like it came from Armadillo had its copyrights added. Stuff which implemented Armadillo-like functionality but was GPU specific was left alone, except for adding the SPDX-License-Identifier at the front.