Add 2D support

[anriseth]

Supports problems with 2D state-space and control-space. Only diagonal volatility matrices. Move towards #3

The policy-iteration approach is best here for large control-spaces.

[codecov-io]

Current coverage is 97.27% (diff: 99.38%)

Merging #7 into master will increase coverage by 2.06%

@@             master         #7   diff @@
==========================================
  Files             4          5     +1   
  Lines           167        330   +163   
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
+ Hits            159        321   +162   
- Misses            8          9     +1   
  Partials          0          0          

Powered by Codecov. Last update b8e23f7...beaba14

[coveralls]

Coverage increased (+2.06%) to 97.273% when pulling beaba14b8158d9100896783bb197b154b53c5cac on 2dnew into b8e23f7746d01a3e85a6547c9764de6b63211f35 on master.