Add a default extrapolation to the input data

[jdha]

Issue:

If you have 1 year's worth of 5 day means it can't process that year without the last 5 day mean from the previous year and one from the next. e.g. if you have 73 time slices from say the year 2000 and you want to produce a BDY file for 2000, you'll need the 73rd time slice from 1999 and 1st time slice from 2001. You can get round this by 'tricking' pyBDY with a pseudo 1999 file containing the 1st time slice from 2000 and a pseudo 2001 file containing the 73rd slice from the 2000 file.

Solution:

By default, copy the first data time slice to t(0)-delta_t and the last time slice into t(nt)+delta_t and throw an error if the timeline requested is outside the range t(0)-delta_t : t(nt)+delta_t