Fix BS1999 scheme

The Burrows & Sharp 1999 flexchem scheme results in NaNs / Infs if the temperature is too low and if LFRic is compiled with single precision. Ian [put](https://code.metoffice.gov.uk/trac/lfric_apps/browser/main/branches/dev/ianboutle/r15119_32fixes?rev=15157) limits on the input temperature and exponents as a temporary patch but this needs to be investigated.