Performance Optimizations + A few Bug Fixes

[mpu-creare]

A few performance optimizations and bug fixes.

This PR also add the following features:

• Compositers.eval_serial: For some compositors, it's important to actually evaluate the nodes in serial for performance reasons. So, even when multi-threading is used, when this flag is True the datasources will be serially evaluated.
• RasterioSource.prefer_overview_closests: when selecting overview levels, we can either select the coarsest overview smaller than the user-specified step size (based on eval coordinates) OR we can select the overview with the closest step size to the eval coordinates... this may be coarser than the specified coordinates. Setting this attr to True will select the closest overview instead of the closests higher resolution overview.

[coveralls]

Coverage decreased (-4.09%) to 85.972% when pulling b1b0222dc3821d9880cc24d29bcf44b79ee453cb on coord-validation into 4c3a3223f47e2b0dabd8240646205518ddfa1a25 on develop.

[coveralls]

Coverage decreased (-4.09%) to 85.972% when pulling b1b0222dc3821d9880cc24d29bcf44b79ee453cb on coord-validation into 4c3a3223f47e2b0dabd8240646205518ddfa1a25 on develop.

[coveralls]

Coverage decreased (-4.09%) to 85.972% when pulling f0a49e4ab946e8f9169d44de1c3a5dcab86a5953 on coord-validation into 4c3a3223f47e2b0dabd8240646205518ddfa1a25 on develop.