Add unit tests for functions in math_utils.py

Add unit tests for functions in math_utils.py

Specifically, this adds tests for:
  TruncatedMean
  Variance
  StandardDeviation
  StandardError

which were not covered before. I also added a couple notes about things
we may want to change in the behavior of the math_utils functions.

Still not covered is PooledStandardError, unit test coverage can be added
for that function in separate CL.

BUG=

Committed: https://crrev.com/bfa797e046fc92508cc5aad6b2fba7547ae33246
Cr-Commit-Position: refs/heads/master@{#305052}

[qyearsley, 2014-11-17 23:15:59]

qyearsley@chromium.org changed reviewers:
+ prasadv@chromium.org

[qyearsley, 2014-11-18 00:57:11]

qyearsley@chromium.org changed reviewers:
+ robertocn@chromium.org

[qyearsley, 2014-11-20 16:57:49]

On 2014/11/17 23:16:35, qyearsley wrote:

PTAL

[prasadv, 2014-11-20 17:52:19]

lgtm

[qyearsley, 2014-11-20 18:16:03]

The CQ bit was checked by qyearsley@chromium.org

[commit-bot: I haz the power, 2014-11-20 18:17:44]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/736573002/20001

[commit-bot: I haz the power, 2014-11-20 18:58:48]

Committed patchset #2 (id:20001)

[commit-bot: I haz the power, 2014-11-20 19:00:25]

Patchset 2 (id:??) landed as
https://crrev.com/bfa797e046fc92508cc5aad6b2fba7547ae33246
Cr-Commit-Position: refs/heads/master@{#305052}