Heat Transfer

[donald-e-boyce]

Adding a heat transfer package that includes anisotropic conductivity.

[pep8speaks]

Hello @donald-e-boyce! Thanks for opening this PR. We checked the lines you've touched for PEP 8 issues, and found:

• In the file polycrystal/heat_transfer/single_crystal.py:

Line 21:5: E303 too many blank lines (2)
Line 37:31: E261 at least two spaces before inline comment
Line 38:25: E261 at least two spaces before inline comment