add support for exporting to TH2

[clelange]

This is a first attempt at addressing #47. I think there are no tests for this in this package, since it's not easy to test with ROOT. I hope I got the axes the right way round. Please let me know what you think. I tried to follow the code style in the class. If this is fine, I can probably extend this further to cover a larger part of the currently unimplemented cases.

I can provide an example notebook.

[coveralls]

Pull Request Test Coverage Report for Build 186

• 0 of 13 (0.0%) changed or added relevant lines in 1 file are covered.
• 16 unchanged lines in 4 files lost coverage.
• Overall coverage decreased (-0.1%) to 50.883%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
histbook/export.py	0	13	0.0%

Files with Coverage Reduction	New Missed Lines	%
histbook/book.py	1	74.92%
histbook/util/init.py	1	58.33%
histbook/calc/init.py	2	87.4%
histbook/expr.py	12	83.09%

Totals
Change from base Build 185:	-0.1%
Covered Lines:	2623
Relevant Lines:	5155

---

💛 - Coveralls

[coveralls]

Pull Request Test Coverage Report for Build 186

• 0 of 13 (0.0%) changed or added relevant lines in 1 file are covered.
• 16 unchanged lines in 4 files lost coverage.
• Overall coverage decreased (-0.1%) to 50.883%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
histbook/export.py	0	13	0.0%

Files with Coverage Reduction	New Missed Lines	%
histbook/book.py	1	74.92%
histbook/util/init.py	1	58.33%
histbook/calc/init.py	2	87.4%
histbook/expr.py	12	83.09%

Totals
Change from base Build 185:	-0.1%
Covered Lines:	2623
Relevant Lines:	5155

---

💛 - Coveralls

[coveralls]

Pull Request Test Coverage Report for Build 186

• 0 of 13 (0.0%) changed or added relevant lines in 1 file are covered.
• 16 unchanged lines in 4 files lost coverage.
• Overall coverage decreased (-0.1%) to 50.883%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
histbook/export.py	0	13	0.0%

Files with Coverage Reduction	New Missed Lines	%
histbook/book.py	1	74.92%
histbook/util/init.py	1	58.33%
histbook/calc/init.py	2	87.4%
histbook/expr.py	12	83.09%

Totals
Change from base Build 185:	-0.1%
Covered Lines:	2623
Relevant Lines:	5155

---

💛 - Coveralls

[coveralls]

Pull Request Test Coverage Report for Build 186

• 0 of 13 (0.0%) changed or added relevant lines in 1 file are covered.
• 16 unchanged lines in 4 files lost coverage.
• Overall coverage decreased (-0.1%) to 50.883%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
histbook/export.py	0	13	0.0%

Files with Coverage Reduction	New Missed Lines	%
histbook/book.py	1	74.92%
histbook/util/init.py	1	58.33%
histbook/calc/init.py	2	87.4%
histbook/expr.py	12	83.09%

Totals
Change from base Build 185:	-0.1%
Covered Lines:	2623
Relevant Lines:	5155

---

💛 - Coveralls

[jpivarski]

This looks good to me and doesn't break any tests (the code coverage going down by 0.1% does not concern me). I'll accept it and increase the version number so that it can go to PyPI.

Thanks!