Improve performance of reading files from disk

[KyleVaughn]

Currently, we use std::getline and std::string related functions for input parsing. However, allocating memory on each invocation of std::getline for each line in a 100 K - 1 M line Abaqus or VTK file is not ideal. Therefore, I think it would be likely be faster to use fgets() and a fixed size buffer for reading files.

The primary issue with this is related to buffer/line size. The buffer must be large enough to handle the max line size. Maybe we just allocate something like 4096 and see how it goes. We do not currently handle files with anywhere near that large of lines.

• gmsh/io.cpp
• mesh/polytope_soup.cpp
• physics/cross_section_library.cpp

[KyleVaughn]

Implemented for all but gmsh/io.cpp, since the files containing a gmsh model are not very large, hence it would not have been worth the refactor effort. Merged to main with #156