improve the look of the track popovers

[fossilfriend]

• [x] edit trackPopover to improve table look and feel; see line 101 in feature_popovers.tsx
• [x] format numbers; set precision to two decimal places;
• [x] ditto for numbers in scientific notation but may need regexp replace to do that; do not convert strings to numeric and back b/c will run into floating point precision errors

[simoncole]

Track popover styling and format improvements

• The line break between data points was extended to the rightmost side of the popover
• The line break color was changed to match the header
• The line break was extended to connect with the popover border and padding was added for the popover content
• A row background highlighting style was added on hover for the popovers to improve readability of the table.
• Numbers in standard notation and scientific notation are now formatted to two fixed decimal places