Add more constants
Add linregress() subroutine and tests
Implement molecular dynamics (MD)

General routines are implemented in the md.f90 module. Example for Argon using
Lennard-Jones potential is in test_md.f90. Diffusion calculation is done in
analyze_md.f90.
Add a notebook for MD analysis
Implement forces for FFT
Merge pull request #41 from certik/md

Implement molecular dynamics (MD)