Improve isharvestable() function for ALMaSS crop model

The data/crops/almass/crop_data_general.csv file now stores a column
`phase_before_harvest` to indicate in which growing phase the plant
can be harvested.  This is `marchfirst` for most crops, and `sow` for
maize.

In theory, with the current setup there could be a problem for
peas/beans if they are planted after March, 1 (the planting dates are
15 February to 15 March).  Planted before March 1st, the last growth
phase before harvest would be `marchfirst`.  But if planted after
March 1st, the last growth phase before harvest would be `sow`.  The
GDD curves for this crop only exist for `sow`, so probably planting
before March 1st should not be allowed for this crop.