Issue870ChartDataFrameMemoryOptimisation

Made primary key field an unsigned 32-bit int (max value 4294967296). Made value fields 32-bit floats once they've been scaled by their multiplier. Refs issue #870

Made primary key field an unsigned 32-bit int that can cope with NaN values. Reduced precision of chart data tests now we're using single-precision values. Refs issue #870

Made precision of dx and mg chart tests more appropriate. Refs issue #870

Updating pandas and matplotlib versions to reflect my test system - these are the latest releases. Refs issue #870

Pandas 1.2.4 requires Python 3.7.1 or above. Refs issue #870

The latest Matplotlib is not compatible with Python 3.6. Downgrading to the newest version that is. Refs issue #870