Increase default memory to 1G for Jalview Webstart and local installations

Jalview's default maximum heap size is 256MB. This is too small for tasks involving genomic variant analysis or reasonably sized alignments such as those employed in Jalview hands-on training.

In advance of any automatic heap setting option, we will increase default to 1G for the 2.10.5 release, and provide a 'small memory' jnlp file (jalview_256MB.jnlp) so users can access the old default if necessary.