Pystata-kernel

Bug fixes:
- No longer install configuration file in custom prefix. 25
- Browse magic now works when only the `in` range is specified. 12

- Drop the temporary Stata program `temp_pystata_kernel_program_name` before its creation. This fixes the `program temp_pystata_kernel_program_name already defined r(110);` error.

- Ported the new error handling mechanism from `nbstata`.
- Better handling of `delimited` (PR 21).
- New magics `%noecho` and `%quietly`.

- New configuration option `missing` controls what should be displayed in the output of the `*%browse` magic for a missing value. Default is '.', following Stata. To defer to pandas' format for `NA`, specify 'pandas'.
- The `SelVar` class now uses Stata's `tempvar` mechanism to create the temporary variable it needs.

*%browse` magic improvements:
- New `in` range option. [Syntax](https://www.stata.com/help.cgi?in) is the same as in Stata.
- Bug fix: variables with missing values are no longer displayed as floats.

*%browse` now correctly display missing values. Previously missing values were displayed as a very large float.