COCOeval.summarize: allow arbitrary maxDets param

[bertsky]

This generalizes the AP summary output such that non-default values for the parameter maxDets become possible:

• upper boundary other than 100 (the default)
• intervals of different length than 3 (the default)
• None instead of fixed number for unlimited (which gets shown as 'all')

Here is an example output for cocoeval.params.maxDets = [None]:

 Average Precision  (AP) @[ IoU=0.50:0.95 | area=   all | maxDets=all ] = 0.419
 Average Precision  (AP) @[ IoU=0.50      | area=   all | maxDets=all ] = 0.468
 Average Precision  (AP) @[ IoU=0.75      | area=   all | maxDets=all ] = 0.421
 Average Precision  (AP) @[ IoU=0.50:0.95 | area= small | maxDets=all ] = 0.183
 Average Precision  (AP) @[ IoU=0.50:0.95 | area=medium | maxDets=all ] = 0.417
 Average Precision  (AP) @[ IoU=0.50:0.95 | area= large | maxDets=all ] = 0.028
 Average Recall     (AR) @[ IoU=0.50:0.95 | area=   all | maxDets=all ] = 0.466
 Average Recall     (AR) @[ IoU=0.50:0.95 | area= small | maxDets=all ] = 0.187
 Average Recall     (AR) @[ IoU=0.50:0.95 | area=medium | maxDets=all ] = 0.516
 Average Recall     (AR) @[ IoU=0.50:0.95 | area= large | maxDets=all ] = 0.045

[HJoonKwon]

@bertsky I thought the same thing and found this PR right away lol. Great!