update niche overlap plot & guidance text

[bjohnso005]

The niche overlap plot function and accompanying guidance text need to be updated. There are new colors not accounted for in the guidance text (and should probably be changed to be R/G colorblind friendly).

Lines 111-120 in espace_nicheov.R ecospat::ecospat.plot.niche.dyn()
col.abn light green: the abandonment niche.
col.unf blue: the unfilling niche. (default green in ecospat)
col.exp red: the expansion niche.
col.stab purple: the stability niche. (default blue in ecospat)
col.pio pink: the pioneering niche.
col.NA grey: the environments outside of both niches.
colZ1 blue line: to delimit the native extent. (default green in ecospat)
colZ2 red line: to delimit the invaded extent.