make accession ID format institution-dependent

[mfrasca]

different gardens use different ID formats for accessions.
in Mompox we just adopted the Bauble standard (year+4.digit) and it served us good,
but here at the JBQ in Quito they already have a 6.digit code for accessions.
at the BTUU in Utrecht they use still an other (much longer) format.

it would be practical if one could specify the accession code format in the institution definition.
the input should then be validated against the format. (maybe an other issue?)

[mfrasca]

This issue was moved to Bauble#59