Keithley2600

_Added_:

- Save time stamps with measurement data.

_Changed_:

- Renamed `ResultTablePlot.update_plot` to `ResultTablePlot.update`.
- Improved documentation of (live) plotting.
- Added `SENSE_LOCAL`, `SENSE_REMOTE` and `SENSE_CALA` to dictionary.

_Fixed_:

- Fixed explicitly defined methods such as `Keithley2600.applyVoltage` not appearing in dictionary.

Added:

- Added `__dir__` proprty to Keithley2600 and its classes to support autocompletion. The dictionary of commands is craeted from the Keithley reference manual.

Changed:

- Remember PyVisa connection settings which are passed as keyword arguments to `Keithley2600`. Previously, calling `Keithley2600.connect(...)` would revert to default settings.

Fixed:

- Explicitly set source mode in `Keithley2600.applyCurrent` and `Keithley2600.applyVoltage`.

This release drops support for Python 2.7. Only Python 3.6 and higher are supported

_Fixed_:

- Fixed a bug in `rampToVoltage` where the target voltage would not be set correctly if it was
smaller than the step size.

Added:

- Keyword argument and attribute `raise_keithley_errors`: If `True`, the Keithley's error queue will be checked after each command and any Keithley errors will be raised as Python errors. This causes significant communication overhead but facilitates the debugging of faulty scripts since an invalid command will raise an error with a descriptive message instead of failing silently.

Fixed:

- Thread safety of communications with Keithley. `Keithley2600Base` now uses its own lock instead of relying on PyVisa's thread safety. See [pyvisa issue 262](https://github.com/pyvisa/pyvisa/issues/262):

> By the way the thread safety simply means that VISA won't crash for order from different threads but there is no lock preventing out of order operations on a single resource. For that you need to use a lock manually.

This version inlcudes some API changes and updates to the documentation and doc strings.

_Added:_

- Accept `range` (Python 2 and 3) and `xrange` (Python 2) as input for voltage sweep lists.

_Changed:_

- Methods `header` and `parse_header` of `ResultTable` are now private.
- Cleaned up and updated documentation.

_Removed:_

- Removed deprecated function `Keithley2600.clearBuffer()`. Use `buffer.clear()` and
`buffer.clearcache()` insead where `buffer` is a Keithley buffer instance, such as
`Keithley2600.smua.nvbuffer1`.

_Added:_

- Added `shape` property to `ResultTable`.
- Added string representation of `ResultTable` which returns the first 7 rows as neatly
formatted columns (similar to pandas dataframes).