Renamed all instances of pymc4 to pymc_tfp

[fonnesbeck]

Naming the repository "pymc-tfp" and the module "pymc_tfp"

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[codecov[bot]]

Codecov Report

Merging #324 (2c6057e) into master (498f0bb) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #324   +/-   ##
=======================================
  Coverage   90.90%   90.90%           
=======================================
  Files          36       36           
  Lines        2847     2847           
=======================================
  Hits         2588     2588           
  Misses        259      259