New discretization function

[artemy-bakulin]

Replaced the old discretization functions with a new one which is more concise and slightly more effective. Done with a 6cc934d commit.

I have tested this new function against the old methods using simulated data. The results are as follows:
| split | hist | discretize (new function)
TNR | 0.93| 0.92 | 0.93
TPR | 0.91 | 0.92 | 0.92

Also changed the order of discretization. Now the expression is discretized after the subsetting.

Added raw_expression attribute which contains unprocessed expression, which is used for subsetting and also in future will be used for estimating the regulator's expression in heatmaps.

[artemy-bakulin]

Fixed problems with "repr" in ExpressionProfile class

59e69e8

[artemy-bakulin]

Then I edited test_run.py in cefa39b

[artemy-bakulin]

Then edited test_expression.py in 4473b4f