Py-opc

Several new additions were made and bugs squashed including:

* updating the readme to reflect frequently asked questions
* adding delays/sleeps in places where they are needed to ensure proper operation of the device
* a new `wait` method was added to allow users to check to ensure the OPC is ready
* error with the `set_laser_power` command was fixed

Add support for the new release of `pyusbiss` which is much more similar to the API of `py-spidev`. This release includes breaking changes if you were using the previous syntax for initiating an OPC object.

A few minor changes were made including the following:
- Users can now choose to use either py-spidev or pyusbiss as a conncetor which allows for the possibility of connecting directly to the Raspberry Pi via a SPI-USB adapter
- `histogram` now defaults to converting the raw histogram into useful units of particles/cc rather than total integrated counts like before
- upgrades were made to the documentation

A few minor changes were made to the API including:
- `_calculate_bin_boundary` was changed to `lookup_bin_boundary`
- `calculate_bin_boundary` was added

Support was (finally) added for calculating the bin boundaries in real values rather than ADC values. Check the documentation for instructions and documentation.

What else was done in this release?
- 32 A version badge was added
- 17 New assertions were added to prevent errors in class initiation
- 5 ADC unit conversion was added

Minor patches to documentation were made with no major changes to the source.

The first major upgrade that breaks old API calls has been released with this version. Key API changes include:
- `read_histogram` is now `histogram()`
- `read_config_variables` is now `config()`

Support for Alphasense OPC-N2 firmware version 18 was added and tested.