enh: adding a criteria to have only candidates with minimum two points

[camilledzt]

IMPORTANT: Please create an issue first before opening a Pull Request.
Linked to issue(s): Closes #357

What changes were proposed in this pull request?

• The filter is now requiring a minimum of two points in the same band to pass.
• Rising alerts must have rate < -0.2 mag/day and last less than 3 days.
• Fading alerts must have rate > 0.2 mag/day in r/i bands, or > 0.5 mag/day in g/u bands.
• The magnitude must be less than 24.
• The cut on the age of the alert is now 5 days (the previous value was 48 hours)

[JulienPeloton]

Very good code -- thanks @camilledzt !