Msieve v1.54

Msieve 1.54:

• Merged the CPU lanczos code from the msieve-lacuda branch, which includes long vector support; compiling with long vectors on a modern machine with a modern gcc has yielded a 10-15% speedup
• Merged fixes by Greg Childers that enable handling singleton and clique removal for datasets larger then 16GB; after years of frustration this finally fixes the 'large dataset bug'
• Fixed errors in MPI block Lanczos (thanks Greg Childers)
• NFS Polynomial selection updates from many people:
  • improved estimate of final e-value
  • improved handling of the factors of leading alg. coefficient
  • reduced verbosity
  • search bounds informed by years of mersenneforum work
• Removed exit() call from error path in NFS filtering (thanks jrcrombie)
• Add support for more modern Visual Studio and CUDA versions (thanks Brian Gladman)
• Made the maximum merge weight configurable (thanks Lionel Debroux)
• Updated the makefile to build GPU code for more recent GPUs
• Updated CUB version
• Added a fix that avoids a crash when linking to GMP 6.2.0 (thanks bsquared)
• Changed the default NFS filtering target density to 90
• Converted readme file to markdown, fixed links (thanks Jupiter)