STYLE: Prefer error checked std::sto[id] over ato[if].

[jhlegarreta]

The ato[if] functions do not provide mechanisms for distinguishing
between 0 and the error condition where the input can not be converted.

std::sto[id] provides exception handling and detects when an invalid
string attempts to be converted to an [integer|double].

ato[if]()

• Con: No error handling.
• Con: Handle neither hexadecimal nor octal.

The use of ato[if] in code can cause it to be subtly broken.
ato[if] makes two very big assumptions indeed:

• The string represents an integer/floating point value.
• The integer can fit into an int.

In agreement with:
http://review.source.kitware.com/#/c/23738/