Release 4.0-beta15

• Speed: reduce overhead in tree operations.
• Models: add relaxed-rs07 that allows rate to vary on different branches.
• Non-equilibrium models now work with variable alignments.
• Syntax: cmd-line functions now look like |w: gy94(omega=w)|
• Syntax: cmd-line functions can now take two arguments like |x y: x + y|.
• Bug fixes:
  • Fix inferring the alignment on a fixed topology (rooted or unrooted).
  • Invalidate compiled modules if the executable changes.
  • Fix consensus-tree and branch lengths ignoring burnin when not specified as percent.
  • Fix (unused) code that integrates out alignments when SPR-attaching on every branch.